Ultramafic rocks are highly unstable in the presence of fluids, giving rise to mineral hydration and carbonation reactions that strongly shape rock properties, the chemical evolution of waters, and the availability of chemical energy required to sustain microbial metabolism. Several new conceptual models exist for peridotite-hosted life on Earth and other rocky bodies in our solar system, such as Europa, Enceladus and Mars. However, there are many questions regarding how, when and where organisms can harness the energy derived from peridotite/water interactions, and what products of biological activity might be produced, in terms of new minerals, organic compounds and gases. In this talk, I will present multidisciplinary efforts designed to assess the key (bio)geochemical processes occurring during modern water/rock interaction in the Samail ophiolite, Oman, and the strategies being used to determine controls on the distribution, composition and function of the subsurface peridotite-hosted biosphere. In particular, I will focus on the mineral transformations that give rise to hydrogen production at low-temperatures, followed by the evidence for biological methane production, even under extreme conditions of carbon and energy limitation.