Hydrothermally-active submarine volcanoes account for much of Earth's volcanism and are mineral-rich biological hotspots, yet very little is known about the dynamics of microbial diversity in these systems. This week in PNAS, Reysenbach and colleagues, show that at one such volcano, Brothers submarine arc volcano, NE of New Zealand, the geological history and subsurface hydrothermal fluid paths testify to the complexity of microbial composition on the seafloor, and also provide insights into how past and present subsurface processes could be imprinted in the microbial diversity.