Mole people

Mole people—people who live at least a significant slice of their lives underground beneath built-up areas, especially in tunnels and heating shafts, but also in permanent underground cities. This definition does not generally include people living normally in the basements of otherwise above-ground houses or city buildings. Mole people may or may not also be considered homeless people. The Teenage Mutant Ninja Turtles franchise has always prominently featured mole people, including the Turtles themselves.

Not to be confused with actual moles—being a mole person has nothing to do with someone's species or appearance, but alludes to their underground-dwelling lifestyle which historically has been compared to that of moles. Mole people can be sentient characters of practically any species.