A mysterious stranger arrives in town. Only working at night, and with no apparent help, he constructs a structure of megalithic stones. The stones are often carved in odd designs, some perhaps with arcane significance. When asked how he moved and worked the stones, the stranger only made veiled references to ancient secrets. The construction lasts nearly thirty years, until the stranger succumbs to a wasting sickness.
Sounds like the background fluff for an adventure, doesn't it? But it happens to be true. While not technically a dungeon, Coral Castle has enough mystery to be the inspiration for a interesting game adventure locale.
Edward Leeskalnin (1887-1951) was a Latvian emigrant who arrived in south Florida in 1919. He purchased a small parcel of land, and spent the next 28 years building a structure of megalithic stones (mostly limestone formed from coral) in homage to his "Sweet Sixteen." How he cut and placed the 1000 tons of stones that make up the structure is mystery. He never let anyone watch him work (he supposedly did much of the work at night, by lantern light), and never had any help or used any heavy machinery that was seen. This secrecy has, of course, allowed a lot of speculation about his methods over the years. Leedskalnin supposedly told people that he "understood the laws of weight and leverage well", and more enigmatically, that he had discovered "the secrets of the pyramids." He also supposedly spoke of using a "perpetual motion holder."
Wikipedia relates the story that a few teenagers reported spying on Leedskalnin and seeing him make blocks of coral to move like "hydrogen balloons." The fact that Leedskalnin published pamphlets on his own theories on magnetism and electricity have helped fuel the wild-eyed speculation. Still, photos exist of Leedskalnin on the work-site with his tools--which are tripods and block and tackle. Of course, maybe that was just to throw people off...
However he did it, Coral Castle is an impressive accomplishment. Almost all the stones are single pieces weighing about 14 tons each. There's a revolving stone door so well balanced that a child can make it turn (or at least could--it stopped working in 1986 and had to be repaired, and now doesn't turn as well) that is made of a 8.2 ton rock. Wikipedia lists its other features: