Today, in the United States, more than 50 percent of the country is in a state of drought, the worst since 1956. While conspiracy theorists might point to the Mayan calendar, the Mayans themselves likely faced similar extreme droughts.

New evidence shows how Mayans adapted to drought conditions to sustain a city with tens of thousands of people for 1,500 years. Research, to be published in the Proceedings of the National Academy of Sciences, points to sustainable water-management technology that helped the Mayan city of Tikal combat drought. But what did this system look like?

Most impressive is a dam that held as much as 20 million gallons of water. The structure, made by human hands, was 260 feet long and 33 feet high and was constructed with cut stone, rubble, and earth. It’s the largest known dam built by the Central American Mayans, according to a news release from the University of Cincinnati, whose researchers were part of the archeological team examining the Mayan water system. But the city didn’t rely on rain falling directly into the huge dam. Instead, they build slanted paved and plastered surfaces with canals that fed the larger reservoirs. Basically, their courtyards and plazas also served as a gravity dam and rainwater runoff was a good thing.

“It’s likely that the overall system of reservoirs and early water-diversion features, which were highly adaptable and resilient over a long stretch, helped Tikal and some other centers survive periodic droughts when many other settlement sites had to be abandoned due to lack of rainfall,” said Ken Tankersley, a co-author of the paper and professor from the University of Cincinnati.

Of course, water purity in cities was an issue then as it is today. To clean up their water using this system, researchers found, the city placed sand boxes in the canals to filter the water before it reached the main reservoir. The researchers found quartz sand, which in not natural to the area, in the ancient city. The Mayans would have had to travel about 20 miles to find the sand for the filtration system.

So what can today’s cities learn from this ancient water management system?

“Water management in the ancient context can be dismissed as less relevant to our current water crisis because of its lack of technological sophistication,” said Vernon Scarborough, a co-author of the paper and a professor at the University of Cincinnati. “Nevertheless, in many areas of the world today, the energy requirements for even simple pumping and filtering devices to say nothing about replacement-part acquisition challenges access to potable sources. … The ancient Maya, however, developed a clever rainwater catchment and delivery system based on elevated, seasonally charged reservoirs positioned in immediate proximity to the grand pavements and pyramidal architecture of their urban cores. Allocation and potability were developmental concerns from the outset of colonization. Perhaps the past can fundamentally inform the present, if we, too, can be clever.”