The sediment under a lake in Mexico contains some of the long-sought answers to the mystery of the Mayan demise.

Ancient Mayans were among the most advanced civilizations of their time. They were some of the first to build cities. They used astronomy to advance agricultural production, and they created calendars and used advanced mathematics. But despite all of their progress, the Mayan empire, built over thousands of years, may have crumbled in just a few hundred.

Scientists have several theories about why the collapse happened, including deforestation, overpopulation and drought. New research, published in Science, focuses on the drought and suggests, for the first time, how extreme it was.

While analyzing sediment under Lake Chichancanab on the Yucatán Peninsula, scientists found a 50 percent decrease in annual precipitation over more than 100 years, from 800 to 1,000 A.D. At times, the study showed, the decrease was as much as 70 percent.

While the drought was known, this study is the first to quantify the rainfall, relative humidity and evaporation at that time. It’s also the first to combine multiple elemental analyses and modeling to determine the climate record during the Mayan civilization’s demise.

Climate scientists use sediment cores to determine the conditions of the past, like geological time capsules.

For this study, scientists examined the layers of mud and clay in the cores from under Lake Chichancanab. During dry periods, the lake volume would have shrunk, said Nick Evans, a graduate student studying paleoclimatology at Cambridge University and first author of the study. As the water evaporated, lighter particles would have evaporated first, leaving behind heavier elements.

“It’s as close as you’ll ever get to sampling water in the past,” Evans said.

Matthew Lachniet, a professor of geosciences at the University of Nevada Las Vegas, who was not involved in the study, said the quantification of the drought is important because it illustrates the power of natural climate variability alone. “Humans are affecting climate,” Lachniet said. “What we could end up with is a double whammy of drought. If you coincide drying from natural causes with drying from human causes then it amplifies the strength of that drought.”