Lost River

The Lost River is a biome. It is a massive, frigid cave biome located deep under the surface and is further divided into seven distinct sub-biomes.

Entrances to the Lost River can be found in both Blood Kelp Zones, the Deep Grand Reef and the border between the Mountains and Bulb Zone. The Cyclops is able to enter the Lost River through all four and traverse the whole way into the Inactive Lava Zone.

Fauna and flora found within the Lost River are immune to the acidic brine, a trait only carried by them. Through oversight, some organisms lack this immunity. Notably the Blood Crawler and Ghost Weed.

Description
The Lost River is a frigid, generally gloomy and foggy biome, possessing a distinct greenish water tint that is mostly illuminated by the acidic brine pools as well as an eerie atmosphere with the occasional Ghostray wails. These brine pools cover almost all of the lower portions of the caverns. The brine pools found throughout most of the cave system will harm the player upon direct contact, with the exception being while within the Prawn Suit, or when within the harmless blue brine found within the Tree Cove.

The Disease Research Facility is located in the Lost River, inside a cavern with its opening in the Junction and the Mountains corridor.

The Lost River is one of the coldest biomes implemented so far, with temperatures on the sea floor rarely rising above 10 degrees celcius.

The Lost River is composed of seven distinct areas:

Bone Fields= The Bone Fields is a vast cavern within the Lost River, where the colossal skeleton of an unknown fauna can be seen laying, giving the Bone Fields it's name. It is connected to the Blood Kelp Trench via the Lost River Corridor, and to the Deep Grand Reef via the Ghost Canyon, each of them having their own flora and particular landscapes. The area also has its own cave system, the Bone Fields Caves.

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Corridor= The Lost River Corridor is the area connecting the Bone Fields to the Blood Kelp Trench. It is mainly populated by Crabsquids, with obscure rock structures covered by Pyrocoral growing on the sides. Gel Sacks and Rubies can be found in abundance scattered around the edge of the acidic brine stream.

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Ghost Canyon= The Ghost Canyon is a large passage featuring multiple caves and tunnels branching out from one huge "canyon". It is connected to the Bone Fields from the south. Numerous roots and bioluminescent-tipped branches litter the sides and bottom of the brine-filled trenches.

The Lost River Laboratory Cache that was set up to study a skeleton can be found here.

This area is also the entrance to the Lost River, from the Deep Grand Reef.

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Ghost Forest= The Ghost Forest is a massive cavern serving as one of the entrance areas of the Lost River (located in the Northern Blood Kelp Zone).

The cavern is greatly populated by whitish glowing flora on the sea floor, as well as dark brown, bioluminescent giant Ghost Trees, and is also the starting point of the brine river. Brinefalls of various sizes are present within the area, and a smaller variation of the gargantuan skull located in the Bone Fields can be found here.

Among the dense vegetation, dangerous predators such as River Prowlers and Blood Crawlers can be found lurking around here. This area is also one of the three locations in the Lost River where the juvenile Ghost Leviathan resides.

An Alien Arch Cache is present in this area, serving as a warp gate to the Primary Containment Facility.

There is also a steep drop off at the back end of the cavern, followed by a long brinefall, that leads to the Junction. From there, the player can either go straight forward into the Bone Fields, or turn right to enter the Tree Cove.

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Tree Cove= The Tree Cove is a large cavern within the Lost River. The brine river forms a large lake there, with the luminous Giant Cove Tree in its center. However, the color of the brine is a bright blue, compared to the corrosive green of the other sections of the Lost River. Unlike the green brine, however, the blue brine does not damage the player upon direct contact.

One end of the cavern opens to the northern passage leading to the Junction, and the other to a corridor leading to the Inactive Lava Zone. Roots and branches grow scattered on the brine lakes and numerous Ghostrays inhabit the chamber. This part of the Lost River is the safest sub biome in Subnautica. This is because the blue brine does not harm the player, there is only one fauna species (which is Passive), and there is no flora which can harm the player besides Deep Shrooms.

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Junction= The Junction is characterised by a large plateau surrounded by a ring of brine. At the top of the plateau are several smokers (hydrothermal vents), with some flora scattered around there. The Sea Dragon Leviathan skeleton can be found here, next to the smokers.

There is also a naturally formed bridge that connects the plateau to the northern side of the cavern, towards the Ghost Forest. Brine falls from the north and south flow into the brine here, which contains some ghost trees and roots. The Junction connects to the Bones Field to the south, the Ghost Forest to the north, and the Tree Cove to the west. The Disease Research Facility and Mountains Corridor are to the east.

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Mountains Corridor= The Mountains Corridor is a large tunnel leading from the Disease Research Facility cave to the Mountains and the Bulb Zone. In-between the DRF Cave and Mountains entrance is the large pit that drops into the Inactive Lava Zone. This cave features many glowing lights on the ceiling, as well as hydrothermal vents and Skeletal Remains. One Ghost Leviathan can be found in the tunnel, above the ILZ entrance.

Trivia

 * Like other segmented biomes (such as the Inactive Lava Zone), the Lost River's technical data divides it into the LostRiver_TreeCove, LostRiver_BonesField, and LostRiver_BonesField again for the Ghost Canyon (this aids the AI in deciding music tracks, random events, and creature behavior).
 * The acid damage from the brine may be possible implementation/reference to the "Pillars of Silence" concept art, where enormous jets of acid dot an alien seabed.
 * The game also considers the actual brine to be a biome. It is labeled LostRiver_BonesField_Lake.
 * The Ghost Forest was originally known as the "Ghost Trees Chamber" before the Bones Update.