Located on the shores of Lake Titicaca in modern-day Bolivia, Tiwanaku was believed by the Incas to be the birthplace of mankind. The city was actually built by a civilization that predates the Incas, but who mysteriously abandoned Tiwanaku about five hundred years before their later neighbors arrived. The origins of Tiwanaku have long been a subject of speculation and wild theories.
The city was built sometime between 300 and 500 AD and abandoned between 950 and 1100 AD. The Inca didn't arrive at Tiwanaku until much later, in the 1400s. When they got there, they found huge rock constructions like the ones in the picture above. (Can you see the little stone faces peering out of the rock?) Instead of admitting that an older civilization than theirs existed, the Inca declared that Tiwanaku had been built by gods and was the place where men had been created. The Incas built their own structures beside the older ones, and today Tiwanaku is a combination of the old and the new. While archaeologists are unsure of the exact functions of each building at Tiwanaku, they are fairly certain it was used as a religious center by the Incas; what its original purpose was is not clear.
Many people have put forward some very odd theories about where Tiwanaku came from: a crash-landing moon, people from Atlantis, or even aliens! Rest assured - Tiwanaku is certainly man-made.
For more on Tiwanaku, visit:
http://www.museum.upenn.edu/new/research/Exp_Rese_Disc/Americas/tiwanaku/index.shtml (the official Tiwanaku Project Site)