Underground \Un"der*ground`\, n. The place or space beneath the surface of the ground; subterranean space.

A spirit raised from depth of underground. --Shak.

Source: Webster's Revised Unabridged Dictionary (1913)
 

Underground \Un"der*ground`\, 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.

Source: Webster's Revised Unabridged Dictionary (1913)
 

Underground \Un"der*ground`\, adv. Beneath the surface of the earth.

Source: Webster's Revised Unabridged Dictionary (1913)