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

Multi-Version Concordance Underground (1 Occurrence) Deuteronomy 8:7 For Yahweh your God brings you into a good land, a land of brooks of water, of springs, and underground water flowing into valleys and hills; (WEB)