The phrase "deep subsurface" refers to the area far beneath the Earth's surface. It includes layers of rock, groundwater, and other materials found deep underground. Full definition
Nematoda from the terrestrial deep subsurface of South Africa. (discovermagazine.com)
Maybe there are deep subsurface microbes producing methane, but maybe not. (discovermagazine.com)
Oil film simulation diagramming hydrocarbon compositions and behavior from both the initial surface slick as well as deep subsurface plumes resulting from the Deepwater Horizon oil spill. (contemporaryand.com)