We live on an alien world. There’s an entire hidden part of planet beneath the surface that we don’t think about. Earth's rock fracture habitat goes down to 5 kilometers, maybe down to 10, and is thoroughly infested with life forms that are dramatically different from the stuff on the surface. So different, in fact, that NASA uses these extreme lifeforms as a guide to its exploration of other worlds in the solar system. Surface life has photosynthesis, but subsurface only a tiny fraction of that energy trickles down. Not a lot of organisms are using organic material, they are processing minerals such as iron and manganese from the rock they live in and could be a model for life on Mars, which gets the most headlines, but Saturn’s moons Enceledus and Titan are both prime candidates, as is Jupiter’s Europa. Even possible cave life on the Moon.