The one word synonym for "underground passageway" is "tunnel".
underground railroad!
passage, alley, aisle, hallway, passageway
alleyway, back street, lane, passageway
chamber, passageway, cave, grotto, crypt
Subterranean does.
the capitol building
gangway. Also passageway, path, lane, passage, corridor, alley
crypt...textropolis
The underground railroad isn't actually a railroad, it's just a term that meant that their activities had to be carried out in secret. The passageway ranged from 14 different Northern states and through Canada. This passageway helped slaves escape.
That is 'passageway' .
Mercenaries, commandos, terrorists, resistance fighters, partisans, underground fighters