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The Maya city of Tikal is famous for its soaring palaces and temples. But something far more humble kept Tikal functioning: its water filtration system, the earliest known of its kind. Researchers recently discovered a volcanic mineral that captures microbes and heavy metals in one of Tikal’s largest reservoirs. Because the material is not found nearby, the finding suggests the presence of a deliberate filter.

The finding contradicts the long-standing idea that the ancient world’s technological prowess was concentrated in places such as Greece, Rome, Egypt, and China, says study co-author Kenneth Tankersley, an archaeological geologist at the University of Cincinnati (UC). “When it comes to purifying water, the Maya were millennia ahead.”

Nestled in the tropical forests of northern Guatemala, Tikal flourished for more than 1000 years. At the height of its prosperity, around 700 C.E., it’s thought to have been home to more than 45,000 people. “It was one of the preeminent Maya cities,” says Nicholas Dunning, a UC geoarchaeologist.

But Tikal’s people had to contend with a dry season lasting roughly from November through April. Storing water in reservoirs was a solution, but that water had to be fit to drink, said Lisa Lucero, an archaeologist at the University of Illinois, Urbana-Champaign, who was not involved in the research. “Keeping water clean was critical.”

A few years ago, Dunning and his colleagues excavated sediments from several of Tikal’s reservoirs. They were surprised to find that one of the largest reservoirs, Corriental, had significantly less contamination from heavy metals, toxin-producing algae, and a mineral associated with fecal pollution than the others. “The water quality at Corriental was much higher,” Dunning says.

Somehow the Maya were filtering Corriental’s water, the team hypothesized. “The Maya used gardens as their bathrooms,” Dunning says. “The water coming into the reservoir would not have been very clean.”

So, the researchers looked closer at the sediments at the bottom of the reservoir. The first hint of an ancient filter was the discovery of quartz crystals. The scientists found four distinct layers, each a few centimeters thick, of brownish, millimeter-scale crystals. (Such sand-size grains can be used for filtering water, but they don’t capture all harmful microbes.) Then, the researchers examined the quartz in greater detail and discovered it was dotted with even smaller crystals of “zeolites.” This type of volcanic mineral can purify water by trapping both microbes and heavy metals within a porous structure, and they’re still in widespread use today, Tankersley says. “Just about everything we drink, from bottled water to wine, is filtered through a zeolite filter.”

The Maya wouldn’t have known about the zeolites in rock, but they would have recognized purifying capabilities, the researchers suggest. A quartz- and zeolite-rich rock formation about 30 kilometers northeast of Tikal is the likely source of the material in Corriental reservoir, the team proposed last month in Scientific Reports. Water at this site “was clear and tasted good,” Tankersley says.

There’s, unfortunately, no direct evidence of what Corriental’s filtration system looked like, Dunning says. However, the team has an idea: Woven reed matting may have held quartz- and zeolite-containing rocks underwater just upstream of the reservoir’s inflow. Such a setup would have been periodically swept away by flash floods following a storm, which would explain the layers of quartz and zeolite found at the reservoir’s base.

The discovery is a potent reminder of the Maya’s technological capabilities, Lucero says. “It shows yet another level of amazement of what ancient peoples accomplished.”