underground

Underground is an antonym of overground.

As a proper noun underground

is (uk|rail transport) the london underground.

As an adjective overground is

above the ground.

As a proper noun underground

is (uk|rail transport) the london underground.

As a noun geosequestration is

geological sequestration, the storage of carbon dioxide in underground geological formations.

As a proper noun underground

is (uk|rail transport) the london underground.

As a noun hypogee is

hypogeum.

As a proper noun underground

is (uk|rail transport) the london underground.

As an adjective hypogean is

existing or growing underground.

As adjectives the difference between underground and hypogeal

is that underground is below the ground; below the surface of the Earth while hypogeal is existing or growing underground.

As an adverb underground

is below the ground.

As a noun underground

is an underground railway.

As a verb underground

is to route electricity distribution cables underground.

As a proper noun Underground

is the London Underground.