Extreme acidophiles (microorganisms with pH optima of < 3) can colonize and exploit subterranean environments, such as abandoned metal sulfide mines, that have the potential for developing widespread or isolated pockets of acidity. Although acidophiles can utilize a wide range of electron donors, inorganic materials (reduced sulfur, ferrous iron, and possibly hydrogen) are often the most abundant sources of energy for acidophiles in the subsurface. The diversity and interactions of acidophilic microbial communities in two abandoned sulfide mineral mines (in Iron Mountain, California, and the Harz mountains in Germany) and a sulfidic cave (Frasissi, Italy) are reviewed. In addition, the contrasting geomicrobiology of two abandoned sulfide mineral mines in north Wales is described. Both are extremely acidic (pH~2) and low-temperature (8–9 °C) sites, but one (Cae Coch) is essentially a dry mine with isolated pockets of water, while the other (Mynydd Parys) contains a vast underground lake that was partially drained several years ago. The microbial communities in these two mines exhibit different relative abundances and often different species of archaea and bacteria. Wooden pit props, submerged in the underground lake, act as a slow-release source of organic carbon in the subterranean Mynydd Parys lake, supporting a microbial community that is more enriched with heterotrophic microorganisms.