Believed to be the center of a vast empire, Teotihuacan was one of the world's largest cities in 600 AD, holding as many as 200,000 inhabitants [source: Metropolitan Museum]. Teotihuacan's lasting centerpiece is a massive boulevard known as the Avenue of the Dead. Stretching for 1.6 miles (2.5 kilometers), the boulevard is flanked by massive platforms, muraled walls, sunken plazas and two pyramids large enough to rival the Great Pyramids of Egypt.
To this day, nobody knows who built Teotihuacan, or what became of its inhabitants. The ancient Aztecs, who lived among Teotihuacan's ruins, were oblivious to the city's original architects -- although they worshiped them for their skills (the word Teotihuacan is an Aztec word meaning "place where men became gods") [source: Sugiyama].
With no royal tombs or artwork depicting the city's leaders, Teotihuacan offers few clues for archaeologists. However, they've uncovered some evidence of the city's violent end. At some point, Teotihuacan began embracing a culture of increasing militarism, possibly as a way to strong-arm nearby groups into paying tribute to the ballooning metropolis. Sometime around the 7th century, brutal conflict erupted, portions of the city were repeatedly sacked and burned, and the ancient city was quickly abandoned.