Teotihuacan, seen from the Pyramid of the Sun.

Teotihuacan, it was massive, one of the first great cities of the Western Hemisphere. And its origins are a mystery. It was built by hand more than a thousand years before the swooping arrival of the Aztec in central Mexico. But it was the Aztec, descending on the abandoned site, no doubt falling awestruck by what they saw, who gave its current name: Teotihuacan, nowadays a famed archaeological site located 45 kilometers N-E from Mexico City, Teotihuacan reached its zenith between 100 B.C. and A.D. 650. It covered 21 square kilometers, and supported a population of more than a hundred thousand.
At the time it was the largest city of the Americas, It had thousands of residential compounds and pyramid-temples comparable to the largest pyramids of Egypt."
Oddly, Teotihuacan, which contains a massive central road (the Street of the Dead) and buildings including the Temple of the Sun and the Temple of the Moon, has no military structures—though experts say the military and cultural wake of Teotihuacan was heavily felt throughout the region.
Until now it is not known which culture built this city, and why Teotihuacan collapsed, or better, why it lasted so long. Mysteries still to be solved.
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