Down the rabbit hole:
The Elemental’s Grotto could be discovered by entering a sinkhole, distubed grave, or just about any subterranean breach; Briarfoot Burrows is another option. My lot crawled down the tunnels beneath Gressfall’s graves, encountering the scenario much sooner than expected.
Holes are dark; bring torches:
Whichever entrance is used, it will open into the side of a vast oval cavern. There are no light sources, so dark or low-light vision would be a bonus. The ceiling is a mass of dangling tree roots; the walls sparkle, but are otherwise featureless. The far end of the area is awash with movement, as hundreds of creatures scoop clay from the walls and form it into glittering spheres. A steady procession of the beings roll the clay down a large passage – much like a dung beetle.
In the centre of the cavern stands a vaguely humanoid form, motionlessly observing the workforce; an earth elemental.
If the elemental detects the group, he will sink down and disappear; five or six of the nearest creatures rush towards the party. Once the PCs are suitably engaged, the elemental will pop up in the midst of the action. It might choose to utter in Terran that surface dwellers should stay on the surface.
The elemental’s assumed face bears a large, red, multi-faceted gem (500gp). Predominately a slab of shale, the smaller creatures’ six legs and two shovel-like claws poke out from below.
As shale monsters are killed off, their numbers will be bolstered by those from the main workforce. Should the elemental bite the dust, then the subservient monsters engaged in combat will become confused and attempt to flee. All other shale critters will stop digging and slowly filter away into the surrounding earth.
What lies beyond:
The only exit is to a side chamber, containing hundreds of the glittering clay spheres stacked floor to ceiling. Whilst heavy and cumbersome, players might choose to take at least some of this material for further examination; something on which I was keen. What might the party do with the clay?
Attempt to identify the stuff with an oh-so-loosely-associated skill check (boo).
If the geologists are nearby, they could easily identify the material as herlic clay, and mention its use to thaumaturgical crafters.
An apothecary or jeweller in Black Mesa could probably identify, and offer to purchase the substance for 1000gp. Be generous!
My party had notions of acquiring the land and starting a mining company, giving up an adventurer’s life and making a fortune. My players are bastards! Had they proceeded with this plan, the goods would certainly not still be lying around for the taking. Not because I want to stymie their creativity, but simply so that they knew something was still working in the depths.