Sediments underlying the world’s oceans and shelf seas cover >70% of the Earth’s surface area. These sediments are the biggest reservoir of organic carbon in the biosphere, hosting over one thousand times more organic carbon than the atmosphere, soils, and seawater combined. Most of this organic carbon was originally produced by organisms, such as phyto- and zooplankton, plants, and animals, that inhabit overlying water and nearby land and whose remains after death were deposited to sediments by sinking through the water column.