dungens
Other words include shrine, chantry, or bethel (seafarer chapel).
subterranean
aquifer
Catacomb
Ground railroad( secret trail)
A Den. Cave. Underground lair
Souterrain Souterrain
Cavern. ++++ More often, just Cave - and the adjective "underground" is superfluous by noun's definition.
Another word for the focus is the EPICENTRE (or if you are American the EPICENTER!) Of course the epicentre and focus are different in that one is on the ground and the focus is underground.
masking, screening, covering, going underground, veiling, suppressing, cloaking, out of sight, concealing.
By defintion, a tunnel is underground.
U N D to create the word 'underground' (UNDergroUND)