User blog comment:Jamintheinfinite/Subnautica Creature Evolution/@comment-35546951-20181012164522/@comment-31440879-20181012214512

uhh sudden release of some carbon dioxide gas due to the whole thing being a giant volcano which killed the creatures and also happened to coincide with a fault forming in the seabed, causing the bodies to sink down into the Earth over time before being uncovered later.

Uh, C02 being released because the game takes place on a volcano (which could cause the leviathans to die). At the same time, a crack formed in the surface, so the bodies sank into this crack. It was later covered (not uncovered), which is how the Lost River was formed.