Although slow to develop after the disappearance here of Olmec influence around 900 BC, the Valley of Mexico saw an explosion of population in later centuries.
Initially the southern part of the valley, where rainfed agriculture was most productive, developed most rapidly.
By 400 BC, a major centre with impressive civic and religious architecture, including a huge truncated conical mound, had emerged at Cuicuilco. In the northern part of the valley, settlement was less intensive, but included Teotihuacan, perhaps selected because of the presence here of a venerated underground cave.
By 100 BC, the development of irrigation agriculture in this northern area had greatly increased its prosperity, and Teotihuacan rivalled Cuicuilco in size. All competition between them, however, was brought to an abrupt halt around 50 BC when Cuicuilco was overwhelmed by lava from the eruption of the nearby Xictli volcano, a disaster repeated several centuries later.
Teotihuacan continued to grow, trebling its population between 100 BC and AD 100, and reaching a staggering 125,000 to 250,000 residents at its peak around AD 300, when almost all the inhabitants of the valley resided there.
