The site that would become Teotihuacan was home to scattered villages as early as 600 BCE, with perhaps 6,000 people in the valley by 200 BCE. What triggered its transformation into a metropolis is partly understood: volcanic eruptions destroyed the rival city of Cuicuilco to the south, and the refugees swelled Teotihuacan's population alongside the economic pull of its obsidian workshops and abundant springs. By 100 CE, the population had reached 60,000 to 80,000. By 300 CE, it had stabilized around 100,000, housed in an estimated 2,000 multi-family residential compounds that accommodated 60 to 100 people each. The city was not organic sprawl. Its planners laid out a central axis -- the Avenue of the Dead, 45 meters wide and 2 kilometers long -- and oriented the entire grid to it. The Pyramid of the Sun, the third-largest pyramid in the world with a base of 219 by 232 meters and a volume of one million cubic meters, anchored the eastern side. This was urban planning on a scale the Americas would not see again for over a thousand years.

Teotihuacan's wealth was built on a brittle black volcanic glass. Obsidian from the nearby mines of Pachuca was the city's most important industry, processed in specialized regional workshops into blades, arrowheads, knife handles, figurines, masks, jewelry, and ornaments. The state monitored trade, movement, and production so tightly that obsidian processing was confined to designated workshops. The resulting tools have been found as far away as Monte Alban in Oaxaca and Tikal in Guatemala, evidence of a trade network that reached across Mesoamerica. The city also developed the talud-tablero architectural style -- an inward-sloping wall surmounted by a rectangular panel -- that spread to Maya sites including Tikal, Kaminaljuyu, and Copan. Whether this diffusion came through trade, diplomacy, or military pressure remains debated, but the style's reach testifies to Teotihuacan's cultural gravity.

Teotihuacan was not a monoculture. Archaeological evidence reveals distinct ethnic neighborhoods within the city, each with its own architecture, artifacts, and burial practices. An Oaxacan neighborhood and a Maya-influenced quarter have been identified, their residents bringing traditions from hundreds of kilometers away. Isotope testing on skeletal remains confirms that many inhabitants were not born locally. The predominant language spoken in the city has been lost, though scholars have proposed early forms of Totonac and Nahuatl as candidates, with influences from Mixe-Zoquean also detected. The city's well-preserved murals -- depicting butterflies, eagles, feathered coyotes, and jaguars in vivid color -- suggest a shared visual culture that transcended ethnic boundaries, binding a diverse population into something cohesive enough to sustain one of the ancient world's great urban experiments.

For decades, scholars assumed foreign invaders sacked Teotihuacan sometime in the 7th or 8th century. The evidence told a different story. When archaeologists looked beyond the elite structures they had initially excavated, they found that the burning was concentrated along the Avenue of the Dead -- the temples, palaces, and administrative buildings of the ruling class. Sculptures inside palatial complexes like Xalla were deliberately shattered. No traces of foreign invasion have been found. The emerging picture is one of internal uprising: a population turning against its own rulers, destroying the symbols of their authority while leaving ordinary residential compounds intact. Drought and resulting famine may have helped trigger the collapse, possibly compounded by pressure from smaller surrounding civilizations. In the power vacuum that followed, regional centers like Cholula, Xochicalco, and Cacaxtla competed for dominance, but none would match the scale of what had come before.

In late 2003, after days of heavy rain, archaeologist Sergio Gomez Chavez noticed a sinkhole near the base of the Temple of the Feathered Serpent. Lowered by rope into the darkness, he found himself standing in a perfectly cylindrical shaft that opened into an ancient tunnel blocked by immense stones. Over the next decade, his team would excavate a passage running 103 meters beneath the pyramid, discovering thousands of artifacts including greenstone figurines, pyrite mirrors, rubber balls, and seashells arranged in patterns that appear to represent an underworld landscape. The tunnel had been deliberately sealed around 250 CE. It joined an earlier discovery beneath the Pyramid of the Sun, confirming that Teotihuacan's builders constructed not just upward but downward -- creating subterranean chambers that may have represented caves of origin, sacred spaces where the boundary between the living world and the world below dissolved.