cave, cavelike, cenote, underground, stalactite, tunnel, mineshaft, passageway, crevice, kiva, catacomb, reservoir, dome, canyon, dungeon, fumarole, labyrinth, volcanic crater, fumaroles, shaft, bathysphere, stalagmites, rockface, carlsbad caverns, crater, waterfall, refugium, airlock, crevasses, cistern, dolerite, hogback, manhole, trench, rock outcrop, ventilation shaft, spelunkers, depths, excavation, grotto, coal mine, sandstone, snowfield, gasometer, crawlspace, sarcophagus, cofferdam, spiral staircase, baptistry, burrow, staircase, lava, snailfish, reservior, tumulus, stairway, cheops, crypt, postholes, slickrock, underwater, glacial boulder, apses, abysses, trackway, cavity, baptistery, sinkhole, oculus, lanternfish, subbasement, manway, hole, tartarus, mountainside, bowels, halite, hibernaculum, atacama desert, esker, tomb, root cellar, hypogeum, bivvy, flume, tabernacle, capsule, moulins, cove, coal seam, kerogen, oxbow, abyssal, igneous rock, geode, limestone, quarry, massifs, basement, crag, lazarette, antechamber, caverns, caves, cellar, lake, foyer, grottoes, hall, hollow, vaults, archway, cirque, corridor, gorge, grottos, pillared, roofless, columned, comfortless, fortress, hallowed, lightless, spacious, storied, unfurnished, cheerless, dilapidated, drafty, roofed, ruined, barricaded, built, castellated

carlsbad caverns national park, cavernous, cavern out, mammoth cave national park, sink, sinkhole, swallow hole

core out, enclosure, hollow out, natural enclosure