Answer: underground

AnswerCrossword Clue
UNDERGROUND

Secret revolutionary force

UNDERGROUND

Clandestine group

UNDERGROUND

Blitz shelter

UNDERGROUND
Escaping detection
UNDERGROUND
London subway.
UNDERGROUNDMaquis, for instance.
UNDERGROUNDBritish subway system
UNDERGROUNDLondon transportation
UNDERGROUNDLike Tubman's railroad