Deep down in the seafloor anaerobic microbes consume large amounts of methane, a potent greenhouse gas. Even though this process is a crucial element of the global carbon cycle, it is poorly understood. Gunter Wegener from the Max Planck Institute for Marine Microbiology and the MARUM, Center for Marine Environmental Sciences, Bremen, Germany, and Jonathan Gropp from the Weizmann Institute of Science in Rehovot, Israel, now found the solution to a long-standing enigma in this process: why methane carbon isotopes behave so differently than expected. In a joint effort with their colleagues Heidi Taubner, Itay Halevy and Marcus Elvert they present the answer in the journal Science Advances.