subterranean

As an adjective subterranean

is below ground, under the earth, underground.

As a noun aflaj is

an irrigation system which catches mountain water and controls its movement down man-made subterranean channels, found in Oman.

As an adjective subterranean

is below ground, under the earth, underground.

As a noun tarpit is

a geological occurrence where subterranean bitumen leaks to the surface, creating a lake of asphalt.