Underground

1. adv. Beneath the surface of the earth.
2. n. The place or space beneath the surface of the ground; subterranean space.
3. a. Being below the surface of the ground; as, an underground story or apartment.
4. a. Done or occurring out of sight; secret.

The word meanings were obtained from OPTED(The Online Plain Text English Dictionary), which is based on “The Project Gutenberg Etext of Webster’s Unabridged Dictionary” which is in turn based on the 1913 US Webster’s Unabridged Dictionary (See Project Gutenburg), as a text file.