Sure, be warned it's just vague ideas and concepts at this point.
So within my ICARUS Universe there is a type of hollow earth thing going on. It's different in that instead of one massive cavernous world within earth, it's a series of caves that connect and disconnect from one another.
Flora and fauna of all kinds from many points in the past fall in, and some manage to eek out a living down there. But I'm still trying to work out realistic, speculative, cave ecosystems for these places.
A lot of the creatures are... not fully thought out I think.