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.