More than a millennium ago in Central America, the great Maya civilization grew, flourished, built beautiful temples -- and then slowly but inexorably collapsed.

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....