Alien Civilization under their planet

I'm trying to trace a story (not sure if it was a novel/novella/webstory and google's keywords are too polluted for my requirements. This is compounded by the fact that the scenarios I remember may not be the core focus of the story.

The story contains a civilization which forms under the surface of a planet (hollow core type) that, over eons, discovers it is under the surface of a planet and then tries to send expeditions to try and uncover the outer surface of the planet.

The problem with these expeditions is that each expedition takes a pocket of air with it and this reduces the civilization's living space in case these are lost. The last viable expedition seems to reach less dense strata but I don't remember what happens next.