What happened? A lot of things, but two scientists now say a key factor may have been nothing more than what they call a "modest reduction in precipitation."
Drought and changing climate have been suggested before as factors that ended the Mayan era, but Martin Medina-Elizalde and Eelco Rohling, a team from the University of Southampton in England, went to the Yucatan Peninsula and figured out ways actually to measure long-ago rainfall. They measured oxygen isotopes in a stalagmite -- a pillar of sediment created by dripping water in a local cave -- and concluded that rainfall in the area probably dropped by about 40 percent between A.D. 800 and 1000.
Forty percent is bad, but modern societies, able to pump up groundwater or build dams, would probably get by. The Maya could not.
"Indeed, the droughts associated with the demise of the Maya Civilizations were not spectacularly dramatic," said Medina-Elizalde in an email to ABC News. "They were strong and persistent enough, however, to have affected the precarious water balance in the Yucatan Peninsula, causing societal problems like warfare, disease, etc., that ultimately led to the civilization collapse."
Rohling wrote too: "How do you think modern civilization would do in the area if water availability were to be reduced by 1/3, assuming that no freshwater pumping existed? I think this is a pertinent question, which illustrates that -- were it not for the pumping and the benefit of (rough) climate predictions -- modern civilization would also be in trouble."
Medina-Elizalde and Rohling, who have published their findings in this week's edition of the journal Science, backed up their isotope data with measurements from local lakes. It's a little like measuring tree rings, but more complicated and more precise.
It shows that there was probably a reduction in the number of major storms coming through the area, and under the hot tropical sun, that probably meant catastrophe for the classical Maya civilization.
"In the Yucatan Peninsula," said Rohling, "there is little surface water, and summer rains collecting in rare natural reservoirs and also man-made storage reservoirs are essential in this highly evaporative region. As soon a summer rains falter, water scarcity builds up."
The resulting crisis, said the researchers, was probably made worse because the Maya -- even with their advanced calendars and record-keeping -- could not see what was happening to them. Modern society, they said, should plan for a changing climate.
"We have knowledge today that the ancient Maya did not have back then. We have important choices to make right now," said Medina-Elizalde.