Word for an underground lake

I'm wondering if there's a good word to indicate something like an "underground lake"? Preferably it would have a connotation of something deep, dark, isolated, murky, mysterious, and a bit archaic.

For example, a writer in the Lovecraftian tradition might wish to say something like:

The fish-men lived for ages in their underground lake, long forgotten by the upper world.

Solution 1:

I was going to say BiscuitBoy is correct - an underground lake is called an aquifer.

Then I was surprised (or maybe not) to discover that Wikipedia has an article titled Underground lake. The author claims that underground lakes may be associated with aquifers or caves.

In fact, underground lake appears to be an accepted term, at least in popular usage. The term subterranean, suggested by JEL, is commonly used in association with underground water bodies.

One more possibility is karst lake. Karst topography refers to areas covered by blankets of limestone, which is easily eroded by water. Such areas may be riddled with caves, seemingly bottomless lakes and underground rivers. Which isn't to say that karst lakes are necessarily completely underground, but you could modify the term - underground karst lake, sunken karst lake, etc.

Solution 2:

If you're after an adjective, 'subterranean' seems as if it might be a good choice:

subterranean, adj.
b. Of an inanimate object: existing, lying, or situated below the surface of the earth; formed or constructed underground, either naturally or by human activity.

["subterranean, adj. and n.". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/193149?redirectedFrom=subterranean (accessed December 13, 2015).]

If 'subterranean' doesn't capture the connotations you want, perhaps Coleridge's choice of 'sunless' would be better:

sun·less (sŭn′lĭs) adj.
1. Being without sunlight; dark or overcast: a sunless moor.
2. Gloomy; cheerless: a sunless life.

[sunless. (n.d.) American Heritage® Dictionary of the English Language, Fifth Edition. (2011). Retrieved December 13 2015 from http://www.thefreedictionary.com/sunless .]

Solution 3:

For location, I would suggest infernal from the Latin infernus, below ground. This has the same meaning as subterranean, but its association is with the underworld, Hades in Greek myth. If you want a more esoteric word (if not actually archaic), you could go with chthonic, meaning "of the underworld" and applied to (often baleful) deities and to Acheron, one of the mythical underground rivers of Hades. Speaking of Hadean rivers, I would recommend Stygian for the "dark and murky" quality. That's the adjectival form of another mythical underground river, the Styx, whose course is dark and gloomy.

Solution 4:

I'd suggest, nethermost and chthonic.

nethermost: farthest down, lowest. M-W

chthonic: something chthonic dwells beneath the earth. This word usually refers to mythological creatures, but you could also refer to your creepy basement as chthonic. Ever heard about the mole men who live underneath the ground in tunnels? Or the mutants who live in the sewers? Or even the Fraggles? Of course, those critters are imaginary, but they're examples of chthonic creatures: beings who live under the surface of the earth. Chthonic beasts are more likely to be demons than angels, so this adjective has a hellish aspect. Many myths feature chthonic creatures. Vocabulary.com