Related by context. Frequent words. (Click for all words.) 70 caverns 65 salt caverns 60 cavern 54 storage caverns 53 geologic formations 52 geological formations 50 rock formations 48 reservoirs 48 shale formations 47 subterranean 46 caves 45 coal seams 45 underground tunnels 45 aquifers 44 helium 44 underground 43 gas hydrates 43 stalactites 43 crypts 43 tunnels 42 underground aquifer 42 ancient tombs 42 shale deposits 42 grottos 42 mine shafts 42 underground aquifers 42 basins 42 dungeon 42 reservoir 41 carbon dioxide 41 cave 41 Caverns 41 tunnels dug 41 passageways 41 swamps 41 sedimentary rock 41 grottoes 40 stalagmites 40 storage tanks 40 methane 40 Bakken shale 40 Ogallala Aquifer 40 vaults 40 Alberta tar sands 39 cellars 39 root cellar 39 peat bogs 39 catacomb 39 labyrinths 39 unmapped 39 underground bunkers 39 dreamscapes 39 coal gasification plant 39 natural gas 38 limestone cliffs 38 wormholes 38 ponds 38 carbon dioxide CO2 38 freshwater lake 38 burrows 38 Saturn moons 38 lagoons 38 vortexes 38 Empty Quarter 38 limestone quarry 38 meltwater 38 storehouses 38 streambeds 38 shale 38 shale gas 38 Marcellus Shale formation 37 methane gas 37 liquid helium 37 lava 37 alcoves 37 underground bunker 37 seawater 37 pipelines 37 IceCube 37 dinosaur fossil 37 ventilation shafts 37 passageway 37 dungeons 37 gas 37 dams 37 cisterns 37 limestone 37 treasure chests 37 catacombs 37 liquefied 37 oilfields 36 Carlsbad Caverns 36 aquifer 36 vats 36 mythical creatures 36 undersea 36 labyrinth 36 calcite 36 quarries 36 trillion cubic feet 36 waterfalls