The Deep Carbon Observatory (DCO) as a whole aims to achieve transformational understanding of carbon's chemical and biological roles in Earth's interior. A multidisciplinary, decade-long effort, the DCO consists of a distributed but closely coordinated set of observational efforts and analytical instruments united by shared databases and a commitment to open access. The program leaders have set ambitious global goals, for example, to reduce the range of estimates of total carbon in Earth's mantle from a factor of twenty to a factor of two, to establish the techniques that resolve ambiguities about possible biotic versus abiotic hydrocarbon production, to accomplish the first global 3-D census of deep microbial life (presented in interactive 3-D!), and to produce a comprehensive database of thermochemical properties and speciation of carbon-bearing fluids and phases at the pressure and temperature conditions of the upper mantle. To meet its objectives, the DCO has organized into four "directorates," three of which-Reservoirs and Fluxes, Deep Energy, and Deep Life-have already been funded through previous Foundation grants. This grant to the University of California, Davis will provide partial funding for two years of operations of the DCO's final directorate, concerned with the most basic physics and chemistry of carbon in the extreme conditions of the deep crust and mantle. When we think of basic natural science, we may recall subjects from high school and college courses such as phase diagrams and equations of state. A phase diagram is a type of chart used to show conditions at which thermodynamically distinct phases (such as solid, liquid, or gas) can occur at equilibrium. An equation of state describes a state of matter under a given set of physical conditions such as temperature, pressure, and volume. These are the subjects of the fourth directorate. One reason so little is known about the deep carbon cycle is ignorance of the basic physics and chemistry of carbon at the pressure and temperature conditions of Earth's interior. Even phase diagrams and equations of state do not exist for relevant carbon-bearing fluids and minerals at the prevailing conditions deep inside Earth. Over the next two years, an international team led by University of California, Davis physicist Giulia Galli will make observations, conduct experiments, and build models concerned with thermodynamics of carbon bearing systems in the crust and mantle, dynamics and kinetics of deep carbon processes, and mineral-fluid interactions under extreme conditions. Its results, such as the database of thermochemical properties, will be essential for the other directorates and for the success of the Deep Carbon Observatory as a whole.