Characterizing the deep biosphere methane cycle is of high importance in light of human alterations to the global carbon cycle. Subseafloor reservoirs of methane have been significant in past climate systems and may soon be quite significant for our present climate. Below the seafloor, multitudes of microbes carry out enigmatic metabolisms. Methane eaters (the ANaerobic MEthanotrophs, or ANME) are suspected to consume ~90% of the methane produced by other deep biosphere microbes. Without this microbial methane filter, fluxes of methane to the atmosphere from the oceans could be much larger. Despite its importance, much about this crucial process remains enigmatic. I work with new tools potentially capable of offering new insights into the subseafloor methane cycle. These techniques, broadly known as “clumped-isotope” approaches, focus on the abundance of methane molecules that have more than one rare isotope substitution. I propose the use of methane clumped-isotope geochemistry to distinguish methane production from consumption and independently constrain the rates of these processes. By measuring the methane clumped-isotope compositions of methane from lab cultures and well-characterized natural samples, I aim to build an isotopologue mass-balance model that can better quantify deep biosphere methane cycling in a variety of environments.