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.
Another word for the London Underground is "the Tube." This term is commonly used by locals and visitors alike to refer to the city's extensive subway system, which serves as a primary mode of transportation across Greater London. The name "Tube" derives from the shape of its deep-level tunnels.
By defintion, a tunnel is underground.