Lake sediment tells of Maya droughts.

Sediment cores taken last year from the bottom of a lake on Mexico's Yucatan peninsula indicate that a series of extended droughts coincided with major cultural upheavals among the Maya inhabitants of the area.

The sediments taken from Lake Chichancanab record climate change in the northern Yucatan for the past 2,500 years, says David A. Hodell, a geologist at the University of Florida in Gainesville. Layers that show higher deposition of minerals indicate times of increased evaporation from the lake.

Between 500 B.C. and A.D. 1000, major dry spells occurred about every 200 years, including a decades-long drought that coincided with the collapse of so-called Classic Maya civilization in the 9th century.

The Maya were especially susceptible to such extended droughts because about 95 percent of their population centers--including all of their major cities--depended solely on lakes, ponds, and rivers containing on average about an 18-month supply of water for drinking and agriculture, says Richardson B. Gill, an archaeologist in San Antonio.

"Sunny days, in and of themselves, don't kill people," says Gill, author of The Great Maya Droughts: Water, Life, and Death (2000, University of New Mexico Press). "But when people run out of food and water, they die."