Underground

Part of speech: noun (n.)

The place or space beneath the surface of the ground; subterranean space. A spirit raised from depth of underground. Shak.

Part of speech: adjective (a.)

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

2. Done or occurring out of sight; secret. [Colloq.] Underground railroad or railway. See under Railroad.

Part of speech: adverb (adv.)

Beneath the surface of the earth.