—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.
Useful english dictionary. 2012.