Related by context. Frequent words. (Click for all words.) 57 underground 46 subterranean 41 underground tunnels 40 tunnels 37 tunneling 36 underground piping 36 injection wells 34 ventilation shafts 34 electrical substation 33 guideway 33 excavated 33 dewatering 33 piping 33 elevator shafts 33 manhole 33 caissons 32 tunnels dug 32 penstock 32 overhead wires 32 underground aquifer 32 kilovolt lines 32 manholes 32 roadbeds 32 excavation 32 tunnel 32 Underground 31 heap leach pad 31 storage tanks 31 cavern 31 slurry wall 31 mine shaft 31 mineshaft 31 lava tube 30 ventilation ducts 30 reinforced concrete 30 trench 30 catenary 30 aqueducts 30 subway 30 excavate 30 caves 30 artesian wells 30 underground workings 30 ledges 29 undersea 29 subsurface 29 underground bunkers 29 aqueduct 29 pipe 29 subway tunnels 29 roadbed 29 kilovolt transmission line 29 tailings impoundment 29 beneath 29 tar pits 29 earthen berms 28 tailings dam 28 rooftop garden 28 parapets 28 Tunneling 28 tramway 28 subway stations 28 basements 28 trapdoors 28 overpasses 28 pipes 28 cisterns 28 stopes 28 dewater 28 mined 28 caverns 28 emplacements 28 underground caverns 28 entombed 27 sewage pipes 27 Harbor Tunnel 27 mines 27 bridge abutments 27 drainage pipes 27 towers 27 subways 27 excavating 27 hermetically sealed 27 crypts 27 Storage Tank 27 elevators 27 spiral staircases 27 trenchless 27 Aperture Science 27 basement 27 vestibules 27 Egyptian pyramids 27 stairwells 27 mine shafts 27 retaining walls 26 coalmines 26 domed 26 viaducts 26 pedestrian walkways 26 dungeon 26 coal seams