(n.) A cavern.

abri acropolis air-raid shelter antre approach trench archives armory arsenal attic bank basement bastion bay beachhead bin blockhouse bomb shelter bombproof bonded warehouse bookcase box bridgehead burrow buttery cargo dock castle cave cavern cellar cellarage chest citadel closet coal bin communication trench concealment conservatory control center countermine coupure cove cover covert coverture crate crib cupboard cyclone cellar depository depot ditch dock donjon double sap drawer dugout dump earth entrenchment exchequer fallout shelter fasthold fastness fire trench flying sap fort fortified tunnel fortress fosse foxhole funk hole gallery garrison garrison house glory hole godown grot grotto hold hole hutch impact area keep lair library locker lumber room lumberyard magasin magazine martello martello tower mine moat mote motte parallel peel peel tower pillbox post potato cellar proving ground rack radar tracking station rath repertory repository reservoir rick safehold safety zone sap sewer shelf shelter slit trench stack stack room stock room storage store storehouse storeroom storm cave storm cellar strong point stronghold subbasement subterrane subway supply base supply depot tank tower tower of strength tracking station treasure house treasure room treasury trench tunnel vat vault ward warehouse warren wine cellar