Terrain scans indicate this biome contains unusually high concentrations of organic and fossilized remains.

― PDA, Dialogue

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

There are four entrances to the Lost River. These can be found in the Northern Blood Kelp Zone , the Deep Grand Reef , the border between the Mountains and the Bulb Zone , and the Blood Kelp Trench . The Cyclops is able to enter the Lost River through all four entrances, and can traverse the whole way into the Inactive Lava Zone . The Seamoth can travel through all passages, but is unable to travel to the Lava zones.

It is the deepest biome that can be explored by the Seamoth , although some sections dip below its maximum depth and the green brine will also wear down its durability.