The Maya ruled central America for 1200 years, creating written language, mathematics, and spectacular architecture.

Then, in AD 900, they mysteriously abandoned their cities, and their empire crumbled. Contaminated water may have played a role.

At their ancient capital of Tikal, 60,000 citizens relied completely on rainwater trapped in reservoirs.

But the rain also washed red mercury paint, which the Maya used to decorate the city, into the reservoirs.

They recognized the problem and devised a way to purify the water—the first people to do so.

They discovered that zeolite—a mineral derived from volcanic ash—could trap heavy metals like mercury in water, and that quartz sand could clarify it.

So, where the rainwater entered one of their largest reservoirs, they built a permeable limestone dam filled with zeolite gravel imported from a nearby deposit and backed by quartz sand held in reed mats.

This system worked so well that, to this day, no traces of mercury have been found in the reservoir.

However, severe droughts in the late 800s may have reduced or evaporated this potable water supply, forcing Tikal’s residents to rely on other reservoirs without the filter system, which are still highly contaminated.

Meanwhile, the Maya people live on: 40% of today’s Guatemalans descend from them. Their zeolite water purification system lives on too, in today’s wastewater treatment plants.