Although we here at the Illuminerdy think Internet conspiracy theories are great places to go for inspiration for your games, there's no reason not to reach back into the depths of history for some deeper mysteries. Old things are creepy things, and creepy things make good game hooks. For a real trifecta, look for old, abandoned things that once housed an advanced civilization -- honestly, it's tough to do better. Although the modern age has leeched some of the mystery from the weird, cyclopean ruins that dot the world, there's no reason to let a little science (in this specific case, anthropology) get in the way of a good game.
A SACRED CITY
On a limestone plateau in the northern region of the Yucatan lie the remnants of Chichen Itza, once one of the most powerful cities of pre-Columbian North America. The remains of the city cover an area of approximately six square miles where hundreds of buildings once stood. Now, most have fallen into such complete ruin that they are merely mounds of dirt, but about thirty buildings can still be seen. Surrounded by dense rainforest, the city was not rediscovered until European explorers stumbled across the overgrown relics of a great Mayan civilization in the mid-1800s, though you, of course, might want to keep Chichen Itza (or an as-yet undiscovered sister city) hidden from prying eyes until your PCs can be the ones to locate it. Of course, you can also set your game in the 19th century so that your PCs can be the first to (re)locate it, or go way back and set your game at a time when the city was still a going concern. Alternatively, use the city as the main location for a Time Travel game, and do all of the above.
WHO BUILT THE CITY?
It's widely assumed that the first stones of the city were laid by a Mayan tribe called the Itza (who help give the city its name) between 500 BC and 325 AD, probably as an agricultural center, though even modern scholarship cannot exclude the possibility that at least some of the structures in the city are even older. The name that survives to the modern day -- Chichen Itza -- can be roughly translated as "The Well of the Itza" (more on those wells, later), but some scholarship suggests that the city was first known by a different name -- the vaguely Lovecraftian "Uuc Yabnal" (which probably translates to "Seven Great Houses" but sounds like something degenerate cultists would chant as your PCs prepare to be used as human sacrifices to dark and forgotten gods). The Seven Great Houses could be of astrological significance -- maybe a reference to a forgotten Mayan Zodiac? But why bother? The numerologicalsignificance alone is probably enough to hang a plot on.
There's also no reason to just accept that the Maya built the structures themselves. You wouldn't be first person to infer that these massive structures were inspired by otherworldly (specifically extraterrestrial) visitation. If you'd prefer a more terrestrial approach, there's a shocking amount of at least semi-serious scholarship that suggests that these early constructions were once part of a vast pre-historic civilization remembered only in the Atlantis legend made famous by Plato, or at least built by refugees fleeing the destruction of their advanced home.
ABANDONED
Regardless, by the end of the Seventh Century AD (another seven!), the city's original inhabitants abandoned the site to move to an area on the western coast of Central America for the next 250 years. It's unclear what drove the Itza away from their city -- they had already built a number of impressive structures in the area -- but it's certainly not out of the realm of possibility to suggest that they were reacting to events predicted by the supposedlyprophetic (and undeniably advanced) Mayan calendar. Maybe they were pushed out by darker things, or an external aggressor -- some of the most violent, terrifying gods of the Maya were said to rule great kingdoms beneath the surface of the earth in a vast land they called Xibalba. Xibalba is pretty scary even as a spirit realm, but if you make it a literal place hidden deep in the Hollow Earth, it's not hard to imagine a hideous army of morlocks bubbling out of the ground to drive Itza out of their homes. In that case, a 250 year absence doesn't seem even close to long enough!
Want more Mesoamerican craziness? Don't worry, dear reader -- there's more to come!