Sinkhole is a third-party (not written or approved by Chaosium) adventure module for the Call of Cthulhu RPG, written by Michael LaBossiere.
From the introduction to the module (and quoted by express permission of the author as per the terms of his website):
"Untold eons ago the earth was visited by strange and terrible beings. Among these was the horrid entity Tulushuggua. Tulushuggua found the water filled crevices of the earth to its liking and squirmed its way deep into the earth’s crust. There Tulushuggua spawned its brood of Tulush who spread out through the dark caverns in search of food.
When man arose he encountered the Tulush. Some men, impressed by the power of the creatures, worshipped them and formed cults. Most of these cults eventually perished, but some lived on in various isolated parts of the world.
Several years ago, a promising young graduate student named Hayes encountered one of these cults and found their activities appealing. With his abilities, Hayes was able to contact more sophisticated cults with a worldwide agenda. Drawn into the madness, Hayes agreed to work for the cults and chose to go back to his beginnings by reviving a Tulushuggua cult. Hayes needed a place to locate his cult and he was able to purchase a plot of land in Wakulla, Florida. Naturally, he selected a piece of land with a sinkhole so that he might contact the Tulush. Hayes selected Janet Reuben to aid him in his endeavors. When Hayes realized he needed help to lure people into his planned cult, he sought Pete Smith, a religious con man. Smith turned out to be something of a disappointment for Hayes, which is where the adventure begins."