Very little is known about the identities and consequences of viruses that interact with the distinctive microbial communities inhabiting oceanic crust. One of the challenges of studying the microorganisms and their viruses there is acquiring sufficient biomass for analysis. In a recent expedition to the Juan de Fuca Ridge Flank, we employed a novel passive filtration system that allowed us to sample the “microbial fraction” (bacteria, archaea) on one filter and the “viral fraction” on another from very large volume of water (10,000 L). This unique large--volume sample allows us to perform analyses that were not possible with previous smaller samples. Specifically, we propose to fractionate the concentrated intact viruses into populations in a density gradient then analyze the fractions (morphology, genome and proteome). Fractionation improves our ability to assemble complete genomes and allows us to link the morphology of the viruses to their genomes and to identify which genes code for viral structural proteins. We will also prepare and analyze a metagenome from the “microbial fraction” to identify intracellular viruses and those integrated into host cell genomes. This will provide an unprecedented level of characterization for the uncultivated viruses and their relationship to their hosts in this remote habitat.