[b]chthonic[/b]. adjective. pertaining to the underworld or spirits of; hell-like.

Another throwback to high school English, this may as well have been Mr. Mike Ireland's favourite word (it was a bonus word on a spelling quiz one time). Chthonian themes of creatures and spirits borne from within the earth or residing within it permeate the Greek culture from which the word originates. Chthonic deities like Gaia, Hades, Persephone, The Erinyes (or "Furies") and even cults to long-dead heroes like Achilles and Theseus had their own unique forms of worship, often involving wine, blood, or other substances poured or buried in the ground. The myths themselves often speak of monsters like Typhon, Echidna, borne of Gaia (the earth) and thus are also considered chthonic. Being that we're talking about the underworld and the earth (as opposed to, say, the "soil" on top of the earth), all things chthonic tend to have themes of life, death and rebirth. 'Cause really, it doesn't get much more old-school than springing to life from the very earth itself.

In usage, chthonic can describe these very forces I've described above. "Hell-like" has become a popular modern-day synonym for this kind of imagery as well. But yeah; underground, underworld, subterranean, maybe even primordial to a degree... that kind of idea:

He bellowed like some chthonic beast of old...

I held my breath and took my first step into the chthonic caverns underneath Silent Hill...

We were lost in that dark, eerily chthonian catacomb for hours!

So how exactly do you say chthonic? Depends who you ask, really. A lot of dictionaries suggest pronouncing the first two letters as a hard "k" sound, while others recommend just considering the "ch-" as silent. If you're big into origins, it used to be "χθόνιος", so use that however you wish.

(Greek origin: khthonios, "of the earth")