Wiktionary
endolith
n. (context biology English) An organism (generally a small or micro-organism) living inside rocks.
Wikipedia
Endolith
An endolith is an organism ( archaeum, bacterium, fungus, lichen, algae or amoeba) that lives inside rock, coral, animal shells, or in the pores between mineral grains of a rock. Many are extremophiles, living in places previously thought inhospitable to life. They are of particular interest to astrobiologists, who theorize that endolithic environments on Mars and other planets constitute potential refugia for extraterrestrial microbial communities.