Many years ago the well was dug deep into the earth. A smooth whole, three feet wide and unknowingly deep. The bucket rose and fell with ease, and the water was clean and sweet, for a while. But something in the earth shifted, and the water became foul, and unwholesome. Livestock refused to drink it, and the crop would wither and grow sickly when watered with it. Slowly, the village moved away from the old well, after several generations it was out of sight, out of mind.
But it was still there; a round whole into the black earth, built up with a low stone lip and capstan for raising and lowering the long since gone buckets. This it would have stayed except for an accident, a youth slipped and fell, plummeting into the darkness. A friend ran and alerted the authorities, and a rescue was planned, except when the would be heroes arrived, the child was no longer in the well. Where once he responded, speaking that he was cold, and wet, but unharmed there was only silence.
A dropped torch revealed the well empty, and a dark place deep, where the torchlight would not penetrate before it was extinguished. The locals balked at such a venture and contacted those more suited for such a task…the PCs.
What Lies Beneath
The earth harbor many things, caverns and underground rivers, lakes and entire seas exist in the lightless gloom. The well almost penetrated a water filled subterranean lake, but there was an event underground. Something shifted, something subsided, and the water table was changed. The bottom of the well was broken open and the water was fouled. Other wells in the area, not so deep, retain the purification of the earth that surrounds them, while the deep well is direct access to the tainted water.
What up with the water? - It has an altered mineral content, it’s been poinsoned by demons, whatever works for your game. The tainted water is why the well was abandoned.
But the water level dropped, and the limestone caverns were dry. There are still plenty of pools of water, as well as deadfalls, hidden pits, and sharp pokey bits to be found. Certainly fun terrain to navigate with a torch in hand and slippery stone underfoot.
Beasties? In a normal ecology, there will be some cave altered insects, white crickets and such, a few winged creatures of the fly variety and of course bats. But this isnt a normal ecology.
Down here, it is hot. The air is sticky with humidity, and it seems to press down. The very weight of the stone and earth above seems to threaten a sudden and violent death. The atmosphere promises a slower and more protracted death by basting and cooking. There could even be a hint of sulfer/brimstone in the thick air.
The trek runs nearly a mile into the earth, and the heat is worse, 115 degrees F and the humidity is nearly 100%. The pools of water are mildly acidic, and hot, akin to falling into pools of urine. Veins can be seen in the stone, flecks of white crystal (qwartz) line the walls where water once ran. It is still running to somewhere deeper in the earth.
Strange things live here. The rocks whisper to each other in the deep grinding tongue of the stone courts. The very earth itself answers in its own deep voice underfoot, all punctuated by the constant dripping of the water funneling down from above.
Choose your path
Things that Should not be - There are unholy things locked away in the earth. Primordial creatures of elastic tar and rage trapped away from the sun and the wind. Alternately there is the classic Balrog demon.
Survival - There is nothing beyond paranoia and normal danger and the PCs must escape before being trapped in the dark. Do they notice the skeletons of others who attempted the trek and failed?
Ooze - The PCs must escape a creature that is the ecosystem, a giant multiton fungus ooze that seeks to devour them with its writhing pseudopods.
The Past - The PCs uncover the ruins/remains of an ancient underground civilization ala Atlantis. There could be mutated survivors, living in the wreckage and gloom. The change in the water was the shell around the city failing, and washing into the heaps of filth that remained, cleaning the city and tainting the water.