Walkthrough (Subnautica)

This page serves as a full-game Survival Mode walkthrough of Subnautica. It also contains gameplay tips and helpful pointers for successfully mastering the oceans. Keep in mind that this page will have spoilers, as it also summarizes the events as you follow along with the story. Note that if at any point in the playthrough you become unable to progress due to a bug, check Console Commands to see if any commands can help. If you are simply looking for exploration tips, check out Exploration Tips or Getting Started.

Background
You play as Ryley Robinson, the Non-Essential Systems Maintenance Chief aboard the Aurora, a spacecraft sent to the Ariadne Arm by the Alterra Corporation to build a phasegate. Following the ship's catastrophic hull failure and subsequent crash landing, he must find a way to escape from the water planet known as Planet 4546B, and discover the cause for the Aurora's crash.

Opening Sequence
After the Aurora's hull failure, Ryley enters Lifepod 5 and ejects the lifepod. It lands in the Safe Shallows, the first biome the player will enter. Following the cinematic sequence, the player will wake up, and after a short struggle to get out of the chair, will stand up. This is when the player will gain control of Ryley. A fire has started in the lifepod, move towards the Fire Extinguisher on the ground, (see Key Bindings for game controls), and pick it up. Aim the fire extinguisher at the base of the fires and use it to put the fires out until you can no longer see their bright red and orange glow. After the fire has been put out you will boot up your PDA, a tablet that will act as a databank, inventory, beacon and screenshot manager, as well as act as your virtual assistant for the duration of the playthrough, giving game hints and anecdotes.

The Lifepod
Inside the lifepod, there is a storage chest with a red top, inside the chest, you will find two Nutrient Blocks and 2 Filtered Waters. These will serve as your food and water source until you can create your own. Along with these, there will be 2 Flares, which can be used to light up dark areas until a more reliable light source is found. Also inside the lifepod is a Medical Kit Fabricator, which will automatically provide First Aid Kits for the player to use when they get injured. It is recommended to take a first aid kit so you have a way to heal if you are attacked by fauna. Also inside the lifepod is the Fabricator, which is used to craft natural resources into other materials, including food and water sources. It is also used to craft several tools, the most vital ones being the Scanner, Repair Tool, Standard O₂ Tank Fins and the Survival Knife. Finally, you will see the Damaged Radio and damaged secondary systems. These can both eventually be repaired using the Repair Tool.

Into the Ocean
There are two hatches on the lifepod, one on the top and one on the bottom, choose one of the hatches and use it to exit the lifepod. At this point, it is important to begin collecting resources to build your basic equipment. The easiest one to craft is the scanner, which requires one titanium and one battery. Begin your search by looking for Limestone Outcrops, they can be found on the floor and walls of several biomes, including the Safe Shallows. These can contain Copper Ore or Titanium, both of which are vital resources early onto the game. Collect at least one of each resource, along with several Acid Mushrooms, which are used to craft batteries. While exploring keep an eye on your Oxygen meter, if it runs out you will have mere seconds to reach the surface before the screen goes black and you die. Once you have collected the appropriate resources, use the fabricator to craft a battery, and then a scanner. The scanner can be used to scan fragments, which will allow the player to unlock new blueprints, and to scan Flora, Fauna and Coral. Once you have crafted your scanner, begin collecting more titanium for a standard O2 tank and survival knife. Gathering Food & Water= In survival and hardcore, keeping your water and food levels high is important if you want to stay alive. The nutrient bars and filtered waters in the lifepod will help you start, but can only get you so far. The earliest sources of food and water both come from small fish, such as the Boomerang, Bladderfish and Peeper. All of these fish can be caught by hand, and cooked with the fabricator to provide a steady food source. However, one fish, the bladder fish, can also be crafted into filtered water, so it is recommended to reserve bladderfish for water. Another source of water, though slightly harder to obtain is Bleach, which is crafted with a Salt Deposit and a Coral Tube Sample. Bleach can be crafted into two Disinfected Waters, which provide slightly more water per bottle than filtered waters. Gathering Resources= There are dozens of different resources in Subnautica, many of which can only be found in certain Harvesting Nodes. Titanium is one of the most important resources in the game, and as such is also one of the easiest to collect. Metal Salvage is a common resource found throughout the map, and, using the fabricator, it can be crafted into four titanium. There are three main harvesting nodes, Limestone Outcrops, Sandstone Outcrops and Shale Outcrop. Each type of outcrop can give different resources, and most biomes only contain one or two types of nodes.

Crafting Important Tools
The survival knife is an important tool, used throughout the game for various reasons. It requires Silicone Rubber to be crafted, which in turn is made from Creepvine Seed Cluster. These are groups of small, yellow balls found on certain Creepvines, a tall, green kelp. Creepvines can only be found in a biome known as the Kelp Forest, which is distinguished by the abundance of creepvines and Stalkers. It is recommended to collect the seed clusters as fast as possible, avoiding stalkers, to minimize the damage taken by an under-equipped player. After collecting the required resources, head back to your lifepod and begin crafting the required materials to make the oxygen tank and survival knife.

The next tool you should focus on is the repair tool, which will be used to repair the lifepod's secondary systems and the radio. Crafting it requires one titanium, one Cave Sulfur and one silicone rubber. The most difficult of these to gather will be the cave sulfur, as titanium is fairly easy to get, and you can use the extra silicone rubber leftover after crafting the survival knife. Cave sulfur can be found in Sulfur Plants, which are found in the Safe Shallows Caves. Each plant contains one cave sulfur, however, the plant is guarded by a defensive fauna known as the Crashfish, which hides inside the plant until the player approaches it, at which point they will be released from the flower and will charge at the player. These fish will swim towards the player at a rapid speed, and after a time explode dealing massive damage to anything in its blast radius. Crashfish can be dangerous to players who lack the proper equipment, however, certain strategies can be used to avoid them. After they launch from the flower, instead of swimming away from the crashfish, swim towards it. Due to its high speed, they are unable to maneuver back towards the player and will swim away, exploding harmlessly. At this point, collect the yellow cave sulfur from the flower and return to your lifepod to craft the repair tool.

Distress Call
Soon after repairing the radio, you will receive a radio message, interact with the radio to listen to it. After listening to the message you will receive a beacon leading to the location of Lifepod 3, and its crew. Swim towards the beacon, watching out for any aggressive fauna, and also keeping an eye on your Oxygen meter. Upon reaching the beacon you will find a destroyed Lifepod 3, with no crew in sight. Explore the wreck and you will find several new items, including a Data Box. Interacting with the databox will allow the player to unlock new technologies, and this specific databox will grant the player the ability to craft the Compass, which can be used for navigational purposes. There will also be an abandoned PDA, these PDAs can be found scattered throughout the world, often containing important information. The final item that can be seen is a Seaglide Fragment, using the scanner, scan the fragment. necessary Fragments= Throughout the world dozens of different types of fragments can be found, these can all be scanned using the scanner and doing so will unlock the item being scanned. Many Blueprints require several of their fragments to be scanned to unlock that technology. Scanning the first fragment will often unlock a Databank entry, giving information about the item. The recipe for the item can only be unlocked once the necessary amount of fragments have been scanned. Wrecks are abundant with fragments, and as such is a good place to unlock new blueprints. Data Boxes= Several blueprints are not unlocked through scanning blueprints, but rather by opening Data Boxes. Unlike most fragments, only one data box needs to be opened to unlock the recipe, although several of the same data boxes can spawn. Abandoned PDAs= Abandoned PDAs can be found scattered across the map, most often in places like wrecks and destroyed lifepods. These PDAs often contain information about the story of Subnautica and can hold codes and clues to help the player. Some PDAs will also provide beacons to important story checkpoints. It is recommended that the player picks up as many of them as possible, to more easily follow along with the story and learn more about the world that surrounds them.

Exploration Equipment
Several pieces of equipment will allow you to go farther, faster and deeper, as well as help you to navigate through the depths. Perhaps one of the most important pieces of equipment is the seaglide, which will allow you to swim at much higher speeds. Unlocking the seaglide requires you to scan two seaglide fragments. If you have already scanned one at Lifepod 3, then you only need to scan one more. Seaglide fragments can be found around the Safe Shallows, in small wrecks known as Scatters. Find one of these scatters and search for seaglide fragments, while you're at these small wrecks, it is recommended that you also search for Beacon and Grav Trap fragments. The grav trap can be a useful tool in the collection of fish for food and water. The beacon is a Deployable that can be placed to mark the location of an important or interesting location.

After crafting the seaglide, two more important pieces of equipment are the Compass and the High Capacity O₂ Tank, which will further increase the player’s Oxygen limit. Both of these recipes require Silver Ore, which can be found in Sandstone Outcrops. These outcrops are most easily found in the Kelp Forest and Grassy Plateaus, which is a flat biome defined by its large rock pillars and red grass. Consider using the seaglide to decrease travel time, as well as using it to avoid predators.

Lifepod 17
Soon after building the seaglide, you will receive a radio message that gives you a beacon to lifepod 17. Lifepod 17 provides a beacon that can help lead the player to the Grassy Plateaus if they are having trouble finding it. Lifepod 17 also leads you to the fragments of an important vehicle, the Seamoth.

Habitat Builder
The Habitat Builder is an essential tool used for building bases. The lifepod cannot fulfill the needs of the player forever, and a more suitable habitat must be built. The habitat builder allows the player to craft several new base pieces that can assist them in their journey, these include Battery Chargers, Beds and Lockers.

Vehicles
Vehicles are instrumental in traversing the world of Subnautica; they can increase your diving depth, travel distance and can be useful tools.

Mobile Vehicle Bay
Crafting any vehicle requires the use of the Mobile Vehicle Bay, a portable crafting station that is used throughout the game. The Mobile Vehicle Bay has unlocked through scanning three fragments, which can be found in the Kelp Forest. Search the Kelp Forest for any Cargo Crates, as several of them contain Mobile Vehicle Bay fragments inside.

Seamoth
The Seamoth is a submersible that can be used to extend the player’s safe diving distance, as well as increase their travel speed. Its fragments can be scanned in the Grassy Plateaus, mainly around wrecks, therefore it is recommended that the player often searches wrecks for fragments. The Seamoth can be used for a majority of the game and can be customized with different Upgrade Modules.

Radiation
Early into your playthrough, the degradation of the Aurora's drive core will cause it to explode. The explosion itself poses little risk to the player, the larger threat is the introduction of Radiation. A rather large part of the map, specifically the area around the Aurora, is now covered in radiation, when the player enters this area, they will take damage. The only way to safely swim near the Aurora is using the Radiation Suit, which, when the mask, suit and gloves are equipped, will negate the effects of radiation. The player must wear all three parts of the suit or they will take continuous damage while in the radiation zone.

Rebreather
The Rebreather is another essential piece of equipment, allowing for deeper dives. When passing 100 or 200 meters the rate at which the player’s oxygen depletes increases dramatically, decreasing the player’s dive time in deeper areas. The rebreather negates this effect and is therefore recommended for all exploration.

Vehicle Upgrades
Although the Seamoth is a very useful exploration tool, it has its limits and can only dive so deep before it reaches its crush depth. To increase its safe diving depth, a Seamoth Depth Module MK1 must be crafted and placed in the Seamoth. Crafting this upgrade, and other upgrades requires the Vehicle Upgrade Console, which can only be built in a Moonpool. Both of these items can be found in the Sparse Reef wreck, the upgrade console as a databox and the moonpool as fragments.

Lifepod 19
Lifepod 19, manned by Second Officer Keen is located in the Deep Sparse Reef, and as such is a great way to mark the location of the Sparse Reef. The wreck located in this biome is found directly above the lifepod, about 100 meters up.

Equipment Upgrades
Also found in the Sparse Reef wreck are Modification Station fragments. The Modification Station is built with the habitat builder and can be used to upgrade several items, such as fins, oxygen tanks and vehicle depth modules. Upgrading items such as the High Capacity O₂ Tank and fins can be very important in the exploration of new biomes, allowing faster and deeper dives.

The Floating Island
After picking up an abandoned PDA in lifepod 19, the player receives the location of the Floating Island, which is the rendezvous point for the Aurora survivors. After investigating the beacon signal given to the player, they discover another abandoned PDA, which reveals the deaths of CTO Yu and Second Officer Keen, the only survivors to make it to the rendezvous. You will then explore the island further, discovering three abandoned seabases, one on each mountain peak, and one in the pass between them. Investigating the bases further, the player will discover several abandoned PDAs, as well as various fragments allowing them to unlock new technologies, such as the Multipurpose Room. The player will also discover a Purple Tablet, a mysterious piece of metal, of unknown origin, scanning it may reveal more information about the item. The PDAs the player picks up will give clues to the player regarding the crew of the Degasi, a small ship that crashed on 4546b 10 years ago. Picking up a PDA will create a new beacon, displaying the location of a second base located 250m below the surface.

The Jellyshroom Cave
Following the beacon given to the player at the Floating Island will lead the player to the Jellyshroom Cave, a cavern located primarily under the Grassy Plateaus, dominated by large, pink, glowing mushrooms, and the Crabsnakes that reside within them. It is recommended that the player stays underneath the jellyshrooms, as approaching the hole in the top will aggravate the crabsnake inside, causing them to jump out and attack the player. Around the base are several fragments, including the Thermal Plant and Nuclear Reactor, which are both viable sources of power for a seabase. Inside the base the player will find several PDAs, further expanding the story of the Degasi and her crew. Take note that several Drooping Stingers hang from the ceiling, and can quickly kill a player who runs into them. There are several sections to the base, including a ladder to an upper portion, scanning items such as the bed and Water Filtration Machine is recommended. Picking up a PDA found in a locker will give the player the beacon to the final Degasi seabase, located 500m deep.

The Deep Grand Reef
The final beacon will lead the player to the Deep Grand Reef, a deep and dangerous biome, filled with aggressive fauna. Creatures such as Crabsquids and Warpers pose a constant threat to the player, so staying vigilant is key. Entering the base, the player will find several PDAs, bringing a conclusion to the story of the Degasi, as well as lab equipment and several fragments. The player will also find a creature egg, this is the egg of a Cuddlefish, a fauna that acts as a pet to the player when hatched inside an Alien Containment, which one of the fragments found in this base. Upstairs, the player will find the data box for the Cyclops Shield Generator, a powerful upgrade for the cyclops. The player will also find an Orange Tablet, another mysterious tablet, similar to the purple one. After exploring this base, the player will finally have seen what happened to the Degasi, and what happened to the survivors.

Rescue Vessel
At some point into the playthrough, you will receive a message from a vessel known as the Sunbeam, a trading vessel run by a man named Avery Quinn. His first message will simply dismiss the Aurora's distress signal as insignificant and not important. However, the ship’s long-range scans will reveal to them the true situation and soon after the player will receive a second message telling them the Sunbeam is coming to rescue any survivors. Receiving the third message will further confirm the imminent arrival of the vessel, until the player receives the fourth and final message, telling them the Sunbeam will land on the planet shortly. It is at this point that the player will receive a beacon to the landing zone and a 40-minute timer will appear, telling the player when the Sunbeam will arrive.

Rescue Inbound
When the player arrives at the beacon, they will find a landmass known as the Mountain Island. They may notice a massive building that appears to be made of an alien alloy, this gargantuan tower is known as the Quarantine Enforcement Platform. As the Sunbeam's arrival counter runs an alarm will sound from the tower, and it will begin to turn until the large pillar is now diagonal to the ground and is pointing skyward. It is at this point when the Sunbeam will become visible in the sky and begin to make its descent. The tower will begin to glow green as if charging up, and a panicked Avery Quinn will tell the pilot to stop the landing and fly back up. Suddenly, a large beam of green light will shoot out of the pillar, obliterating the vessel.

If the player fails to be at the landing site when the Sunbeam lands, this dialogue will play.

Alien Technology
Following the loss of the Sunbeam, the player is encouraged to explore the Quarantine Enforcement Platform, the large tower responsible for destroying the vessel. Upon approaching it, they will discover the Forcefield Control, a machine that requires a purple tablet to activate. Using a purple tablet they have crafted, or found on the Mountain Island, placing it inside will unlock the forcefield blocking their way, allowing you to enter the building. Exploring the base will reveal two Alien Relics, the Alien Rifle and the Doomsday Device, along with another Purple Tablet and three Ion Cubes. Once the player reaches the top of the ramps, there will be an elevator shaft down, jumping into it will bring the player to the lower section of the building.

In the lower section, there will be a moonpool, where the player can park their vehicles, or use them as another entrance. There will also be another forcefield, which the player can deactivate with a purple tablet. Behind this door is the deactivation terminal, and interacting with it will reveal that the player is infected with a disease and will give a databank entry revealing the location of the Disease Research Facility.

Radio Message
Approximately 5 days after the rescue attempt by the Sunbeam, the player will receive a message from Alterra HQ, telling them that there is a blueprint for an escape rocket in the Aurora. They will be given the code 2679, which will unlock the Captain Quarters.

Equipment
The following equipment is recommended and/or required for exploring the Aurora:

Radiation Suit (required to approach the Aurora)
 * Stasis Rifle, for self-defence from Cave Crawlers, Bleeders, and possibly Reaper Leviathans.
 * Repair Tool, for repairing the Drive Room and mending broken doors.
 * Propulsion Cannon, for removing crate barriers at both the main hallway and Laboratory entrance, and to remove the Bleeders from the player's arms. A Repulsion Cannon is also a viable tool for removing crates and barriers. They could also be used for self-defence against Cave Crawlers and bleeders.
 * Fire Extinguisher, for various fires (some can be found on site)
 * Laser Cutter, for entering various rooms including the Prawn Suit Bay and Aurora Black Box Terminal.
 * A Seaglide for quick navigation through the submerged sections.
 * A Survival Knife / Thermoblade to kill the Bleeders.
 * A Grav Trap can be used to quickly contain the Bleeders in the Drive Room.

Entering The Aurora
The Aurora is located on the Crash Zone a murky biome inhabited by several Reaper Leviathans, which pose a serious threat to the player. However, approaching the vessel using a specific path will allow you to avoid these hostile faunas. A Seamoth can safely sneak past by approaching from the Safe Shallows nearest to the Aurora and hugging the Aurora's side, until reaching a gap large enough for the Seamoth to squeeze through. It is recommended to disembark in shallow water so that the ramp beneath you will protect you from a Reaper attack, though the Reaper doesn't normally swim that close to the entrance. Go up the ramp, watching out for Cave Crawlers, and picking up the items in Supply Crates. when you reach the top walk across the metal beam on the right side, which will bring you to a hallway, which will allow you to enter the Aurora.

Cargo Bay 3
This section will serve as a general walkthrough, focusing on major story locations in the Aurora, for a more detailed explanation see, Aurora. Down the left corridor, there is rubble blocking the entrance to Cargo Bay 3. Before this, an Abandoned PDA named "Aurora Engineering Drone - Log" next to a single Propulsion Cannon Fragment, right at the start of the ramp leading to the entrance. These can be gotten around dropping down from above or by using the Propulsion Cannon or Repulsion Cannon to move these out of the way. There is then a locked door with a keypad that can be interacted with. Entering the code from the previously mentioned Data Download into the keypad will open the door. The code is 1454.

Prawn Suit Bay
On the left there will be a room labelled Locker Room, inside are several items that can be useful to the player, such as batteries and first aid kits. Continuing through the room will lead the player to a sign labelled "Prawn Bay", inside this room, there are several fragments for the Prawn Suit. To the right is a pile of rubble with broken Prawn Suit Fragments and other miscellaneous things piled on top of each other. The fragments may require nearby fires to be put out to be scanned. Among the rubble is a Storage Module, located on an upgrade console. Next to that is a ramp blocked off by fire and rubble. Once those are cleared, there is a door that leads to the "Living Quarters".

Living Quarters
The Living Quarters are various rooms that provided living conditions for the various crew and passengers on the ship. To the left, you can find Cabins 1 through 3 and the Captain's Quarters. Cabin 1 is locked by a code, which is "1869", found in the Abandoned PDA named "Sweet Offer". In this room, a Blue Cap, Arcade Gorge Toy, and an Abandoned PDA, "Responsible Autonomous Relationships", can be found. Another locked door at the end of the hallway, Captain's Quarters, can be unlocked with the code "2679", which was code received on the radio. Inside the room, there is an Aurora Miniature, an Aurora Poster, an Abandoned PDA titled "Captain's Log" on the bed, and a Data Terminal that contains a Data Download titled "Alterra HQ - Last Recorded Transmissions", that unlocks the Neptune Escape Rocket.

Going Deeper
The Seamoth is a fast and useful vehicle, which can help the player in several situations, however, even with its most advanced depth module, it can only bring you so deep. Therefore crafting more advanced vehicles is important for any deep exploration.

The Prawn Suit
The Prawn Suit is a versatile, mechanical walker, equipped with two customizable arms, and a jump jet used for vertical movement, as well as a small, upgradeable storage locker. Its strong, reinforced hull can withstand high pressures, and its arms are powerful enough to crush rock. Unlocking the Prawn Suit requires you to visit the Aurora's Prawn Bay (see here for more info). The basic Prawn offers little in terms of mobility and versatility, however, the Prawn's customizable arms allow it to perform more specialized tasks.

There are five prawn arms available:


 * The basic Prawn arm allows the player to pick items up, and punch things such as outcrops.
 * The Prawn Suit Drill Arm allows the player to mine Large Resource Deposits
 * The Prawn Suit Grappling Arm allows the player to swing around, increasing movement speed and mobility, as well as allowing the player to hang from places that would otherwise be hard to reach.
 * The Prawn Suit Propulsion Cannon is functionally similar to the Propulsion Cannon, allowing the player to pick up and launch items from afar
 * The Prawn Suit Torpedo Arm allows the player to launch Vortex Torpedo or Gas Torpedo.

All of these arms can be crafted with the Vehicle Upgrade Console, along with any other Prawn Suit upgrades, such as the Prawn Suit Jump Jet Upgrade, and can be equipped with the Prawn's upgrade slot, located on its left shoulder.

The Cyclops
The Cyclops is a massive submarine, measuring 55 meters long, large enough for the player to walk around in, it primarily serves as a mobile base, as the construction of Interior Modules and some Miscellaneous Items, using the Habitat Builder, is possible within the vehicle. It also comes with several small storage lockers, providing some space for the player to store items. Another perk the Cyclops provides is its Moonpool, which grants the player the ability to store and transport a Seamoth or Prawn Suit using the Cyclops.

Constructing the Cyclops
The Cyclops may have a complex crafting recipe, however, the true challenge lies in scanning and unlocking the blueprint for the vehicle. There are three separate blueprints required to unlock the Cyclops and three fragments of each blueprint must be scanned, (however only one recipe is needed), they are the Bridge, the Hull and the Engine. All three of these parts can be found throughout the world, though not all in the same place. The first piece we will focus on is the Engine. One of the easiest ways to find several engine fragments is in the Aurora, in Cargo Bay 3. If not enough fragments are found there, the Underwater Islands wreck is another viable option. The Bridge and Hull fragments both spawn in several of the same places, one of the quickest ways to find enough fragments for both is the Sea Treader's Path wreck, where both of these fragments spawn in high quantities.

The Cyclops, like all other vehicles, has several upgrade modules available, these upgrades include several depth modules, increasing its safe depth, a Cyclops Shield Generator, temporarily making the vessel invulnerable, at the cost of power, and the Cyclops Thermal Reactor Module, allowing the Cyclops to recharge its power in high heats.

Lost River
The Lost River is a large, subterranean biome, located under a large portion of the map. It is easily identifiable by its hazardous, green brine.

Finding the Lost River
There are four entrances to the Lost River, located in the Blood Kelp Zone, both the Northern biome and the Trench, the Deep Grand Reef and on the border of the Bulb Zone and the Mountains. All of these entrances are large enough for a Cyclops submarine to fit through, though some may be a tighter fit, making it harder to navigate through. If the player wishes to bring a Cyclops submarine down, it is recommended to use either the Bulb Zone entrance or Northern Blood Kelp entrance, as both of these are large enough to fit the Cyclops through, with plenty of room to navigate. If the player simply wishes to bring the Prawn suit, or even swim down without a vehicle, using the Bulb Zone or Blood Kelp Trench entrances is recommended, as both of these biomes contain less hostile fauna, allowing for a safer trip. Using a vehicle is recommended, as it provides a near-limitless source of oxygen for the player. Furthermore, building a Seabase in the Lost River may be prudent, as it can save the player time taken for trips up to the surface. For this reason, using a Cyclops may be a better option, as it has significantly more storage than a Prawn Suit or the player.

Exploring the Lost River
The Lost River is a dangerous biome, filled with hostile fauna and deadly brine. Safely exploring these caverns requires the proper equipment and knowledge to prevent an untimely demise. Perhaps one of the most noticeable features of this biome is the bright, green brine, found on the seabed throughout the area. Touching it will cause the player to lose health, and prolonged exposure can cause death, the exception to this is the blue brine found in the Tree Cove, as touching that is completely safe. The safest way to explore under the brine is in a Prawn Suit, as neither the player nor the vehicle will take damage, and using the Claw Arm, the player can pick up items they may find under the brine. The Prawn also serves as a defence against many creatures, taking less or no damage from them. There are several aggressive faunas in the Lost River, of all sizes. The largest of which are the three Juvenile Ghost Leviathans, posing a serious threat to the player. The player should also watch out for River Prowlers and Blood Crawlers, along with the occasional Crabsquid.

Disease Research Facility
The Disease Research Facility is a Precursor base located in the Lost River Junction. Its location is marked by a Sea Dragon Leviathan Skeleton, as the head points towards the facility. The interior of the Disease Research Facility is accessible via an entrance on its northeast face near the top of the structure. It is mainly a straight path from the entrance to the Warper room, with some deviation. Starting at the entrance, the right-hand wall contains an Ion Cube and a force field room that can be unlocked with a Purple Tablet; this room contains a Data Download entitled "Alien Research Data". Continuing along the path, which turns left and downwards, there are another Ion Cube and a small tank on the left. The main path turns right into a room containing the Sea Dragon's egg and various fauna. The path turns right again, with a large aquarium on the left, accessible via a broken window, housing the skeletal remains of Research Specimen Theta. At the end of the path is a room with various Warper parts, including a torso that can be scanned. A data terminal in this room, when interacted with, triggers the PDA to prompt the Player to perform a self-scan, which reveals the Kharaa Bacterium has spread to the skin on the player's hands.

Inactive Lava Zone
The Inactive Lava Zone is a subterranean biome found under the Lost River, and only accessible through the Lost River. It is characterized by its glowing magma and a reddish hue. The biome often reaches temperatures upwards of 50°C, which can damage the player, so wearing the Reinforced Dive Suit is advised. Additionally, there are four types of fauna in this biome that pose a threat to the player, 2 Sea Dragon Leviathans, Lava Lizards, Warpers and Lava Larva, the last of which will not damage the player but will attach to their vehicles and slowly drain its power, so bringing extra Power Cells is recommended.

Entering the Biome
There are two entrances to the Inactive Lava Zone, one in the Lost River Tree Cove via the Inactive Lava Zone Corridor, located behind the Giant Cove Tree, and one in the Mountains Corridor, in a large pit that leads directly down to the biome. Both of these passages are large enough to fit a Cyclops Submarine, however, they lead to different places in the biome. The entrance via the Mountains Corridor will lead you to the western side of the biome and the Tree Cove entrance will lead you to the eastern side of the biome, there is not much of an advantage to one entrance or the other, however, the eastern entrance will lead you almost directly to the Lava Castle. Be careful when entering this biome, as a Cyclops Depth Module MK2 and/or a Prawn Suit Depth Module MK1 is necessary to explore the biome. Throughout the biome there will be many Large Kyanite Deposits, which must be mined using the Prawn Suit Drill Arm, alternatively, the player can find single nodes of Kyanite in the Lava Castle, however, this is a much more dangerous method of collecting the resource. Kyanite will be an important material for the next level of Depth Modules for both the Prawn and the Cyclops.

The Lava Castle
The Lava Castle is a sub-biome within the Inactive Lava Zone and is also the location of the next Precursor Base. The Lava Castle can be found in the centre of the biome, and it is a huge structure of volcanic rock possessing a rather large interior cavern, obsidian pillars, and thermal vents. Various cave systems lead to the center of the castle, which is a large chamber featuring a lava pool with tall spires of rock around its edges. The structure seems to be the breeding grounds of Lava Lizards as it is the only place their eggs can be found. A Sea Dragon Leviathan swims near the Lava Castle, so taking caution is advised. The entrance to the Lava Castle is marked by two pillars that appear to be made of the same alloy as the Alien Bases. Finding them is not a difficult task as they glow green, contrasting with the red of the rest of the biome. The inside of the biome is similar to a maze, and it can be quite easy for the player to get lost, therefore bringing extra food and water is advised, as well as a Pathfinder Tool, to help mark the way. Once the player finds the centre they will find the Alien Thermal Plant.

Alien Thermal Plant
Before entering the Alien Thermal Plant, be advised that two Purple Tablets are necessary to unlock the full facility, and doing so will give the player access to several story-related items that are required for progression.

Behind the first door locked door is a blue Alien Tablet, which could be another important Alien Artifact. Behind another door is the main generator for the facility, along with three data terminals, one of which unlocks the Blueprints for the Ion Battery and Ion Power Cell. There is also an Ion Cube Pyramid located inside, which can be mined with the Prawn Suit Drill Arm, providing several ion Cubes. Inside another room is a Principal Alien Arch, linking back to the Quarantine Enforcement Platform, this can be activated with an Ion Cube.

Lava Lakes
The Lava Lakes is the final biome of Subnautica and is located at 1300+ meters deep. It is also the location of the Primary Containment Facility, the final Alien Base of the game. There are two entrances to the Lava Lakes, one called the Lava Falls, which can be found near the Lava Castle. It is several "lava falls", all leading deeper underwater until you reach the Lava Lakes. The other Entrance is located in the Lava Pit, a large pit found on the outskirts of the main chamber. The walls of the pit are covered in large grooves, at the northern edge of the pit one of the grooves acts as an entrance to the Lava Lakes. The biome is quite small, however, it houses another Sea Dragon Leviathan, and due to the size of the biome, avoiding it is very difficult. Additionally, the temperature of the biome rarely drops below 75°C, so staying within your Prawn Suit or Cyclops is advised. At the end of the Lava Lakes is the Primary Containment Facility and found in the biome are five Sonic Deterrents. Entering the facility requires a Blue Tablet, though there is a small entryway that is accessible beforehand, and can protect the player from aggressive fauna, as well as provide oxygen.