Anasazi Cliff Palace at Mesa Verde
The Cliff Palace is something of a mystery wrapped in a conundrum. Built by the Anasazi (now termed Ancient Puebloans) in the 13th century AD the Cliff Palace is perhaps the most well known structure of its type in the United States. This self-contained village rests beneath a massive sandstone outcrop overlooking the valley below.
It's not really known why the Anasazi - a Navajo word translated either as the "Ancient Ones" or more ominously as "Enemy Ancestors." - abandoned the Cliff Palace in the years between A.D. 850 and A.D. 930 but it's thought that the harsh winters and sweltering summers - and the more frequent drought conditions - made sustainable farming especially problematic. Of course, there's plenty of scope here for a little speculation as to why such a structure (with all of the hours invested in building, etc) would be abandoned - plague, famine, pestilence, war or perhaps even aliens!