(n.) The nether world (according to classical mythology, the abode of the shades, ruled over by Hades or Pluto); the invisible world; the grave.

Bassalia Gehenna Sheol Tophet abysm abyssal zone arroyo bathyal zone benthos bottom waters bottomless depths box canyon breach break canyon cavity chap chasm check chimney chink cleft cleuch clough col coulee couloir crack cranny crater crevasse crevice cut cwm deep deepness defile dell depth dig diggings dike ditch donga draw excavation fault fissure flaw flume fracture furrow gap gape gash gorge groove ground gulch gulf gully hades hole hollow incision inferno inner space joint kloof leak mine moat netherworld notch nullah ocean bottom ocean depths ocean floor opening pass passage pelagic zone perdition pit profoundness profundity quarry ravine rent rift rime rupture scissure seam shaft slit slot split the deep the deep sea the deeps the depths trench underworld valley void wadi well workings yawning abyss