adv., adj., n., & v.
adv. 1 beneath the surface of the ground. 2 in or into secrecy or hiding.
adj. 1 situated underground. 2 secret, hidden, esp. working secretly to subvert a ruling power. 3 unconventional, experimental (underground press).
n. 1 an underground railway. 2 a secret group or activity, esp. aiming to subvert the established order.
v.tr. lay (cables) below ground level.