No one knows who the killers were or what they sought. Booty apparently did not interest them. Some 3,600 pieces of jade, including several jade boulders, were left untouched; household goods in the palace and ceramics in Cancuén's giant kitchen were undisturbed. But to archaeologists who have dug up the evidence over the past several years, the invaders' message is clear. By depositing the bodies in the cistern, "they poisoned the well," says Vanderbilt University archaeologist Arthur Demarest. They also chipped the faces from all the carved likenesses on Cancuén's stone monuments and pushed them over, facedown. "The site," says Demarest, "was ritually killed."

CANCUÉN WAS ONE OF the last major dominoes to fall in the Pasión River Valley, part of the ancient Maya heartland in present-day Guatemala. Many other cities had already met similarly decisive ends, and throughout the southern lowlands of Mesoamerica, what came to be known as the collapse of the Classic Maya was well under way. The civilization that had dominated the region for 500 years was sliding into a prolonged, irrevocable decline.

While warfare obliterated some vibrant city-states, others simply faded. The kuhul ajaw, or holy lords, who had celebrated their every deed in murals, sculpture, and architecture, no longer commissioned new works. Public displays of hieroglyphic writing became scarce, and dates in the Long Count calendar system all but disappeared from onuments. Population fell drastically. Nobles abandoned palaces and squatters moved in, lit cook fires in the old throne rooms, and built lean-tos next to crumbling walls. And then even the squatters left, and the jungle reclaimed what remained.

Elsewhere in the Petén lowlands of Guatemala and in southern Mexico, the collapse took longer. Even as Cancuén fell, rulers of the great city-state of Tikal in the northern Petén were building ceremonial structures. But 30 years later Tikal's population began to drop precipitously as well. Its last dated monument was inscribed in 869. By 1000, the Classic Maya had ceased to exist.

The question has fascinated scholars and the public since 19th-century explorers began discovering "lost cities" in the Petén: How could one of the ancient world's great civilizations simply dissolve?

Early speculation centered on sudden catastrophe, perhaps volcanism or an earthquake or a deadly hurricane. Or perhaps it was a mysterious disease, untraceable today—something like the Black Death in medieval Europe or the smallpox that wiped out Native American populations at the dawn of the colonial age. Modern researchers have discarded these one-event theories, however, because the collapse extended over at least 200 years. "There isn't any single factor that everybody agrees on," says Southern Illinois University's Prudence M. Rice.

Scholars have looked instead at combinations of afflictions in different parts of the Maya world, including overpopulation, environmental damage, famine, and drought. "You come away feeling that anything that can go wrong did," says Rice.

They have also focused on the one thing that appears to have happened everywhere during the prolonged decline: As resources grew scarce, the kuhul ajaw lost their divine luster, and, with it, the confidence of their subjects, both noble and commoner. Instability and desperation in turn fueled more destructive wars. What had been ritualized contests fought for glory or captives turned into spasms of savagery like the one that obliterated Cancuén. Says Simon Martin of the University of Pennsylvania Museum: "The system broke down and ran out of control."