Acidic, metal-rich waters generated by the microbially accelerated dissolution of pyrite and other sulfide minerals, are frequently encountered in derelict mine sites, including many that have been long-abandoned. While these waters are major causes of environmental pollution and are toxic to the majority of prokaryotic and eukaryotic organisms, some life forms (mostly bacteria and archaea) thrive within them. "Acidophiles" comprise a surprisingly wide diversity (in terms of both physiology and phylogeny) of microorganisms. This article reviews current knowledge of the distribution and biodiversity of this group of extremophiles.