The Architects

The Precursor Race was a highly advanced and ancient space-faring alien race that arrived on Planet 4546B approximately one thousand years ago, searching for a cure for the Kharaa Bacterium. Most information surrounding them is currently unknown. The ulterior motive of the Precursors was to stop the Kharaa Bacterium from destroying their race, which was a bacterium that they discovered on an unknown planet. Their plans failed and Kharaa was released onto 4546B.

Biology
No Precursor has ever been seen, so nothing is known of their physical appearance. It is likely that they were a lot larger than humans based on the size of their bases and the objects within them, a prime example being the height of the surgical tables found in the Disease Research Facility which come up to just below Ryley Robinson's neck. For use as a surgical table they would most likely be around the same height as the surgeon’s waist, by this it can be estimated that the Precursor race would have averaged between seven and eight feet in height.

They are known to have had an extremely long lifespan, as evidenced in a data bank entry by the mention of a ninety-six year old Precursor being considered very young. The same individual was near 700 Earth-years old (born 1,708 Earth-years ago) when its body became infected after the Kharaa outbreak and the consciousness was stored in a data hub. It is unknown if this lifespan is naturally long, or augmented like Paul Torgal’s, who had expected to live to the age of 160 prior to crashing on 4546B.

The use of the word "seed" in reference to the unborn form of a Precursor possibly implies plant-like biology, though it is equally possible that seed could refer to an animal ovum. Imperfect translation may also be the reason behind this. The language clearly does not perfectly translate into English as shown by the naming of the Sea Emperor Leviathan. Though this theory of plant-like biology is not supported by the fact that no flora species in game appears to be visibly effected by Kharaa, and the deactivation terminal in the Quarantine Enforcement Platform that takes a blood test.

Description
Although the Precursors are not seen in-game, they were an extremely advanced race - able to create advanced technology (such as teleportation devices, weapons capable of bringing down large ships, wirelessly powered biomechanical life forms capable teleporting at will, of etc.) capable of capturing a Sea Emperor Leviathan. They are also capable of creating Data Hubs, which they can upload their mind to, the digital equivalent of a medically-induced coma. There is only evidence of a single Precursor doing this, who were uploaded to the Dunes Sanctuary, with their body disposed of shortly afterwards.

The Precursors were able to create and use Warpers as guards for their bases and use them to hunt down creatures infected with the Kharaa Bacterium, and create technologies far more advanced than humanity's current technological state, and at the same time, predating humans by thousands of years.

The Precursors are also able to create "keys", known as Alien Tablets, which unlock various Precursor machines, including forcefields and Alien Arches, which function as teleporters. The Artifacts also possibly served as a personal computer and security clearance of sorts to its user according to the entry for the Blue Tablet. In addition, they can create alloys that are highly heat-resistant and can last over one thousand years with no sign of degradation. Ion Cubes are another creation, having been created from an unknown element not found on the Periodic Table, and fabricated inside the Primary Containment Facility's Antechamber. Most, if not all Precursor technology is seemingly powered by Ion Cubes. Interestingly enough, Ion Cubes appear to be used in ceremonial fashion, as the three Sanctuary Caches have multiple Ion Cubes adorning them. The Precursors also created the Alien Robots that repair and, if need be, defend the Alien Bases from intruders.

Arctic Events (Story events of Subnautica: Below Zero)
The Precursors had a presence in the planet's arctic regions as well as the crater, and Precursor structures will be present in Subnautica: Below Zero. At least one base in the region is known but their overall purpose in the region remains unknown.

Crater Events (Story events of Subnautica)
Around 1000 years ago, after exploring a planet for routine expansion, the Precursors came across the Kharaa Bacterium, and, most likely discovering it's lethality and indirectly causing it to infect their species, go to various planets in order to develop a cure. One of the outposts they set up for research was on 4546B, and there, the Precursors installed the Disease Research Facility, Alien Thermal Plant, and Quarantine Enforcement Platform to assist them in their research. The Disease Research Facility serving as a means to test the bacterium on native creatures and to construct and repair their enforcers, the Warpers. The aforementioned creature was a biomechanical creation of the Precursors, having taken a creature and augmented it. Their purpose was to keep the bacterium's spread to a minimum. During the construction of this facility, it is likely the Precursors discovered the Gargantuan Fossil and the Ancient Skeleton, with the Precursors setting up a laboratory near the remains of the latter.

The Alien Thermal Plant served to power the remaining Precursor Bases. The Precursors had discovered 4546B's natural thermal energy, and installed the plant within the Lava Castle as a means of utilizing it. The Quarantine Enforcement Platform was constructed to prevent any unauthorized ship from landing or leaving. Among the bases, the Precursors built three Sanctuary Caches in the Deep Sparse Reef, Northern Blood Kelp Zone, and inside a meteor crater within the Dunes. These Caches served as a failsafe should the bacterium be unleashed onto the planet. Inside the Caches, the Precursors installed Data Hubs as a means of keeping them alive when their physical body could no longer be used.

After testing on multiple creatures, the Precursors discovered a Sea Emperor Leviathan specimen that was immune to Kharaa due to a unique digestive enzyme it produces, dubbed Enzyme 42. The Precursors then constructed a fourth base, the Primary Containment Facility in the Lava Lakes biome in close proximity to the Alien Thermal Plant for the sole purpose of containing a Sea Emperor Leviathan specimen for research. They obtained a 1600-year old Sea Emperor Leviathan that had recently laid seven eggs, implying they must have ambushed the creature when it came to lay its eggs. Through unknown means the Sea Emperor was transported to within facility's aquarium. However, it is revealed to them that their captive is too old to produce a stable variant of the enzyme, the weaker enzyme produced by it only capable of causing remission of Kharaa. The Precursors then turned to its eggs, deducing that the young Emperors would produce a more stable enzyme. However, they could not figure out how to make the eggs hatch. The Precursors then took one of the eggs and dissected it, although this resulted in the death of the fetus and it was unable to produce the enzyme. The Sea Emperor Leviathan tried to communicate with the Precursors using its own telepathy, attempting to tell them that it's children could not be forced from their eggs, and they must hatch when ready. Despite the Precursors themselves using telepathy, they were unable to hear the Sea Emperor Leviathan's telepathic speech. At some point a second egg was removed from the seven and placed within the facility's egg lab.

After discovering the possible relation between the Sea Emperor Leviathan species and Sea Dragon Leviathans, the Precursors took an egg from a Sea Dragon Levviathan and planned on studying it, hoping to discover how the Sea Emperor Leviathan eggs hatched without losing more of their precious Sea Emperor eggs. However, the parent of the stolen egg tracked it down to the Disease Research Facility, and rammed into the base's support cables, causing severe physical trauma to the Sea Dragon Leviathan's head and causing major structural damage, killing the seven Precursors within and indirectly causing the previously contained Kharaa Bacterium to be released into the ecosystem. This attack caused the Precursors to initiate an emergency quarantine. This act also forced the Precursors to lock down all Arches leading to the Primary Containment Facility. The Sea Dragon Leviathan died of its injuries nearby the base.

Despite their best efforts to minimize the bacterium's spread, it is unsuccessful and Kharaa rapidly infects 4546B and themselves. Facing extinction and unable to make the Sea Emperor Leviathan eggs hatch, the Precursors decide to preserve their consciousness' in Data Hubs, although there is only direct evidence of a single Precursors doing this. Soon afterwards, it can be assumed that the Precursors went extinct, with only their research and structures to be found in the future.

Contact With Other Species
The Precursors are known to have scavenged technology from other races. All but one of the relics held by them were taken from other races, some of these races even more ancient than themselves. One of the relics is a sword from Earth, showing that the Precursors visited the planet and possibly even made contact with humans at some point. Inside the Sparse Reef Sanctuary the PDA notes similarities between Precursor architecture and old Earth religious iconography. This may be coincidental, however it may indicate that the Precursors had an influence on the formation of ancient human religions and societies. If this is true then it is also likely that some ancient Earth languages were based on the language of the Precursors; the PDA is capable of translating it without ever having encountered the language before, if the language was from a language family completely foreign to any on Earth then translation should be impossible. Their written language bears slight resemblances to Hebrew.

Their language can be heard from the deactivation terminal within the Quarantine Enforcement Platform, after Ryley attempts to disable, and after he successfully disables. The PDA is able to translate the former

Trivia

 * Various Data Downloads suggest that the Precursors may be capable of some form of telepathy.
 * The ancient Mongolian blade found in the antechamber of the Primary Containment Facility suggest that the Precursors visited Earth in the past. The Alien Carving also supports this theory, as the yin and yang symbol was a major part of ancient Chinese philosophy.