The Lark will come across an anomalous environment on her journey, referred to as a "suspended ocean". The body is under a constant flow between sublimation and deposition (changing between a state of gas and solid without becoming liquid between transformation). The gas becomes thicker as the "suspended ocean" reaches the bottom, where it is solid.
But what makes this environment defy known chemistry, is that the mixture of gases is the regular atmosphere. Its states of matter have no observable chemical or physical catalyst for this change in state. It is breathable to terrestrial creatures.
At a certain depth, the gas is thick enough to suspend creatures like the gigantic Ripple-Fin Eels. It is here that the Lark plays a special game of tag with creatures like this. She flies at a level where there is no difficulty breathing, and then dives into the thicker atmosphere. When she is present, all the creatures in the entire ocean put their food chain on hold to play with her. The game is that they go at a fair pace until they're tagged. Anything tagged helps propel the Lark towards un-tagged creatures, or ambushes them for her to catch. The game lasts a few hours, until the lark is on her way to the salt marshes.