abri air-raid shelter antre approach trench asylum bolt-hole bomb shelter bombproof bunker burrow cache cave cavern communication trench concealment corner countermine coupure cove cover covert coverture cranny cubby cubbyhole cyclone cellar dark corner den ditch double sap dugout earth entrenchment fallout shelter fire trench flying sap fortified tunnel fosse funk hole gallery grot grotto hideaway hideout hidey hole hiding hiding place hole lair mine moat niche nook parallel recess refuge retreat safety zone sanctuary sap secret place sewer shelter slit trench stash storm cave storm cellar subterrane subway trench tunnel undercovert warren