From the late eighth through the end of the ninth century, something unknown happened to shake the Maya civilization to its foundations. One by one, the Classic cities in the southern lowlands were abandoned, and by A.D. 900, Maya civilization in that region had collapsed. The reason for this mysterious decline is unknown, though scholars have developed several competing theories.
Some believe that by the ninth century the Maya had exhausted the environment around them to the point that it could no longer sustain a very large population. Other Maya scholars argue that constant warfare among competing city-states led the complicated military, family (by marriage) and trade alliances between them to break down, along with the traditional system of dynastic power. As the stature of the holy lords diminished, their complex traditions of rituals and ceremonies dissolved into chaos. Finally, some catastrophic environmental change--like an extremely long, intense period of drought--may have wiped out the Classic Maya civilization. Drought would have hit cities like Tikal--where rainwater was necessary for drinking as well as for crop irrigation--especially hard.
All three of these factors--overpopulation and overuse of the land, endemic warfare and drought--may have played a part in the downfall of the Maya in the southern lowlands. In the highlands of the Yucatan, a few Maya cities--such as Chichén Itzá, Uxmal and Mayapán--continued to flourish in the Post-Classic Period (A.D. 900-1500). By the time the Spanish invaders arrived, however, most Maya were living in agricultural villages, their great cities buried under a layer of rainforest green.