cave limestone sinkhole stalactite lava canyon quarry karst hollow grotto volcanic hole manhole tunnel underground mineshaft cistern excavation cavity reservoir waterfall crawlspace burrow basement oxbow sandstone crypt cove trench crevice dungeon weathering caving enclosure chalk dolomite gypsum bed fault wormhole solvation subterranean stalagmite caverns carpark kirkyard outcropping riverbed outcrop aquifers attic aquifer pond cellar ledge dank pit cromlech berm disused tailings hillock geyser vaults moat excavate overlooks jetty riverbank cisterns catacombs kents volcano crevices vents geysers waterfalls pyramid culvert puddle holey mine excavator cinder seafloor watermill hydrothermal shallows beneath floodplain dweller graveyard boathouse skylight lavra fetid outcrops alluvial necropolis magma