Definition of underground
(adj)
under the
level of the ground; "belowground
storage areas"; "underground caverns"
conducted with or
marked by
hidden aims or methods; "clandestine
intelligence operations"; "cloak-and-dagger activities
behind enemy lines"; "hole-and-corner intrigue"; "secret missions"; "a
secret agent"; "secret
sales of arms"; "surreptitious
mobilization of troops"; "an undercover investigation"; "underground resistance"