Goblin Well

From the introduction:

On the outskirts of town (any town) lies the old well house, a simple shelter with no walls and a thatched roof. The well is very old, and the wellhead and shelter have been rebuilt several times in the well’s history, though the shaft has never been maintained and the stones lining it have begun falling down the shaft into the aquifer below. Unbeknownst to the townsfolk, this reveals several natural caverns that intersect the well shaft. A 10 gallon bucket is raised or lowered by a loop of chain draped over a rounded crossbeam in the roof.