The subsurface of both marine and terrestrial environments has been rising a lot of interest lately, more since it was shown that its biogeochemical cycles are more active than originally thought. The Iberian Pyritic Belt (IPB), one of the largest sulfide deposits on Earth, is situated the southwestern Spain. It is the source of the highly acidic Tinto River, maintains a low pH (mean 2.3) and a high metal concentration, especially Fe(III), along almost its whole length. The high acidity predates the extensive mining of this area, being caused by the activity of the chemolithotrophic microbial community found in the subsurface, whose main source of energy is the oxidation of the pyrite with release of Fe(III) and sulfate. The geomicrobiology of the IBP subsurface is poorly studied. A recent drilling project has shown the existence of methane deep in the subsurface, and, together with the geochemical data, it points toward microbial methanogenesis. This proposal aims toward a multidisciplinary and interdisciplinary approach to study the geomicrobiology of methane production/consumption in the IPB subsurface. A broad suite of tools from geology, microbiology, geochemistry, mineralogy and molecular ecology will be used to achieve the following: (i) to determine where in the subsurface are methanogenesis and methanotrophy taking place; (ii) to determine the methane sources and sinks; (iii) to identify and characterize the microorganisms involved into the methane cycling, their energy sources and electron acceptors, and the metabolic pathways employed; (iv) to characterize the microbe-mineral interactions associated with the methane cycling, and (v) to develop a geomicrobiological model for the methane generation and consumption and its relevance for the C cycle in subsurface. This proposal is in line with the international trend in research toward the understanding of the subsurface environments, and it will contribute to the excellence of the European Research Area.