Terrestrial subsurface geomicrobiology is a matter of growing interest on many levels. From a fundamental point of view, it seeks to determine whether life can be sustained in the absence of Sun radiation. From an astrobiological point of view, it is an interesting model for early life on Earth, as well as a representation of life as it could occur in other planetary bodies. Río Tinto is an unusual extreme acidic environment due to its size (100 km), constant acidic pH (mean pH 2.3), elevate concentration of heavy metals and high level of microbial diversity, mainly eukaryotic. Río Tinto rises in the core of the Iberian Pyritic Belt (IPB), one of the biggest sulfidic ore deposits in the world. Today it is clear that the extreme characteristics of Río Tinto are not due to acid mine drainage resulting from mining activity, but to the chemolithotrophic microorganisms thriving in the high concentration of metal sulfides of the IPB.