While the caverns of Earth isnt really lifeless, but as far as i know, only small beings can sustain themselves with the scarce food supply. What can be the hardness level of putting big animals into the underground caverns of an alien biosphere? I wondered about the plausibility of the following options : Maybe their biosphere could rather rely on heat energy of underground thermal waters than light? Maybe enough biomass could reach the underground from the surface the feed big fish and amphibians? Maybe bacteria could convert heat energy to light to feed regular plants? Should they feed on coal?