Underground

01 n. The place or space beneath the surface of the ground; subterranean space.

1.
The place or space beneath the surface of the ground; subterranean space.
“A spirit raised from depth of underground.” — Shak.
2.
a subway or subway system, especially in the United Kingdom.[chiefly British]
3.
a secret organization opposed to the prevailing government; as, the French underground during the Nazi occupation.
4.
a group or movement holding unorthodox views in an environment where conventional ideas dominate, as in artistic circles.

02 a. Being below the surface of the ground; as, an underground story or apartment.

1.
Being below the surface of the ground; as, an underground story or apartment.
2.
Done or occurring out of sight; secret.[Colloq.]

Phrases & compounds

Underground railroad — See under Railroad.

03 adv. Beneath the surface of the earth.