Funds from this grant provide two years of continued support to the Reservoirs and Fluxes community of the Deep Carbon Observatory (DCO). Questions about quantities and movements of deep carbon are fundamental to the DCO. How much carbon do the core, mantle, and deeper crust contain? Where is it? What mechanisms move carbon within and across Earth’s layers, and what are the rates of these movements? Deep carbon’s movements are also consequential for humanity, as when deep carbon erupts to the surface through volcanoes, or seeps out of the seafloor as hydrocarbons, or belches out when tectonic plates slip across one another, contributing to tsunamis. Now numbering more than 110 members, the Reservoirs and Fluxes community has matured into a set of networks addressing these and other questions, including mysteries of carbon’s most precious form, diamonds. Over the next two years, this international scientific network will focus on making important discoveries across five areas: the degassing of deep carbon through volcanoes; the degassing of deep carbon through tectonic and other diffuse processes; the origin, age, and depth of diamonds and the mineral inclusions within them; the fluid dynamics of carbon transport in volcanoes, and the global circulation of carbon between Earth’s surface and core; and the chemical forms, mineral hosts, and reactions of carbons moving between reservoirs. Supported activities include the establishment of the first global network for direct measurement of Cox flux, production of a database on eruptions and volcanic gases, the construction of an international reference collection of diamonds for research, and the development of new geodynamic models of deep carbon circulation.