The Deep Carbon Observatory launched in 2009 with an ambitious plan to understand how carbon inside Earth—deep carbon—contributes to and affects the global carbon cycle. Carbon is one of the most important elements of our planet: carbon-based fuels provide much of our energy; carbon is an essential element of life; and excess carbon in our atmosphere presents one of the greatest planetary challenges of our time. The amount of carbon in the easily accessible surface environment, however, is only a tiny fraction of the carbon in Earth. Before DCO, remarkably little was known about the physical, chemical, and biological properties of Earth’s deep carbon.