Summary Earth's microbial biosphere extends down through the crust and much of the subsurface, including those microbial ecosystems located within cave systems. Here, we elucidate the microbial ecosystems within anthropogenic 'caves'; the Iron‐Age, subterranean tombs of central Italy. The interior walls of the rock (calcium‐rich macco ) were painted ~2500 years ago and are covered with CaCO 3 needles (known as moonmilk). The a...