aquifer
Cavern. ++++ More often, just Cave - and the adjective "underground" is superfluous by noun's definition.
geo and thermal
inground
A pool is a place you can go; a pool is a thing in which to swim.Distinctions of this kind are not set in concrete, the important thing is to know that the word pool is a noun.
The noun 'pool' is not specifically a collective noun but it is a word that will lend itself quite well to use as a collective noun in a suitable situation, for example a pool of bathers, a pool of fish, a pool of workers, etc.
A geothermal pool is called a reservoir.
The heating of underground water by magma is called geothermal activity. The word geothermal comes from the Greek geo meaning "Earth" and therme meaning "heat".
Thermal comes to mind. Geothermal is another.
subterranean
Billiards.
pool, lagoon
Crypt.
Crypt.
A Den. Cave. Underground lair
Pond, pool.
A kitty. A fund. A pot.
Geothermal energy is heat from the Earth.