The brilliant red that colored Maya temples, tombs, and ritual objects carried a hidden cost — one recorded in the mud of nearby lakes. Mud from two lakes on Mexico's Yucatán Peninsula shows mercury pollution reached toxic levels and persisted for centuries during the height of Maya civilization, according to a study published Nov. 4 in Science Advances.

The researchers, led by Auburn University paleolimnologist Matthew Waters, link the contamination to cinnabar, a mercury-sulfide mineral ground into the pigment vermilion. "Our study shows that modern toxicology and pollution regulations were far exceeded at the most toxic levels for centuries around Coba and Xcach," Waters said, according to Reuters.

The finding adds to a growing view that pre-industrial societies could cause lasting environmental damage. Here, the damage was not from mining or burning fuel but from a sacred pigment washing off painted surfaces into the water people lived beside.

Lake Mud as a Pollution Archive

Lake bottoms are natural archives. Each year, a thin layer of sediment settles out of the water, trapping pollen, microscopic organisms, and chemical traces. Drilling a core through those layers and dating them lets scientists read environmental history like pages in a book.

Coring lakes is also a way around a basic archaeological limit. Excavations show where people lived and what they made, but lake sediments reveal what flowed off the landscape year after year, including chemicals people never intended to release.

The team collected cores in May 2023 from Lago Cobá, beside the Maya city of Cobá, which may have held 50,000 to 90,000 people, and from Cenote Xcach, a sinkhole lake about 100 meters (330 feet) wide near a little-excavated ruin, Archaeology Magazine reported. The cores span more than 2,000 years.

Two other Yucatán lakes, Polol and Chichancanab, showed no comparable mercury rise. That control matters: it argues against pollution blown in from distant volcanoes or regional sources and points to local human activity at the two city sites.

Levels Above Modern Sediment Benchmarks

Mercury pollution coincided with the Preclassic and Classic periods, roughly A.D. 40 to 820, and the most polluted samples reached 2.3 parts per million (ppm), Live Science reported. At Cobá, a spike began around A.D. 250, and levels fell back to about 20 parts per billion by A.D. 520. At Xcach, contamination continued until the Maya collapse.

Comparing ancient mud with modern rules requires care. The best match is sediment guidelines, and a widely used toxic effect threshold for mercury in sediments is 1 ppm, meaning the peak exceeded it more than twofold. Live Science also cited limits for drinking water and fish, but those apply to different materials and are not direct comparisons with lake mud.

The ecological signal is clearer. "We measured multiple biological markers in the sediments for bacteria, algae, cyanobacteria, shelled organisms and insect midges, which all decreased or vanished in the lakes," Waters said, meaning the mercury reached multiple levels of the food chain. Freshwater shellfish fossils were largely missing from polluted layers.

An Imported Pigment With a Toxic Pathway

Cinnabar does not occur naturally in the Yucatán's limestone. The researchers think it was traded in, probably from deposits in present-day Honduras, making it a valuable, sacred commodity associated with blood and life. Red ochre, a common alternative pigment, gets its color from nontoxic iron oxide.

Once washed into lakes, inorganic mercury can be converted by bacteria in oxygen-poor bottom mud into methylmercury, which builds up in fish and the animals that eat them. That is the likely pathway to harm, but the team did not directly measure methylmercury, Archaeology Magazine noted, so its levels in Maya-era waters remain an inference.

"They would have seen a strange color change in the water ... and a drop in its productivity, such as fish for protein," Waters said. "They experienced this for decades and, in one case, for centuries."

The scale surprised the researchers. Waters told Live Science the team "did not suspect" the Maya used cinnabar "in the quantities that we were seeing." He also emphasized the culture behind it, saying the pigment "added beauty and culture to these people," whose achievements in science, construction and art were extensive.

A Contributor, Not the Cause, of Maya Decline

The study does not claim mercury toppled Maya cities — the authors are careful on that point. Drought, deforestation and soil erosion are leading suspects in the eighth and ninth century collapse, and at many sites mercury damage had declined centuries earlier. Waters said contamination "may have been a contributing factor" by degrading water quality and food supplies.

The research builds on earlier work, including a 2020 University of Cincinnati study that found toxic mercury and blue-green algae in reservoirs at Tikal in Guatemala. "We were able to find a mineral fingerprint that showed beyond a reasonable doubt that the mercury in the water originated from cinnabar," said UC's Kenneth Tankersley at the time. A 2022 review found seven of 10 Maya sites tested had mercury at or above modern environmental toxicity benchmarks. What the new study adds is a long, dated record from natural lakes and evidence of ecosystem collapse inside them.

The lesson resonates today. Legacy mercury persists for centuries in sediments, and modern sources from coal burning to small-scale gold mining continue to load lakes worldwide. U.S. readers can check state fish consumption advisories, which are largely driven by mercury, before eating locally caught fish. Pregnant women and young children face the greatest risk, because methylmercury can affect the developing nervous system.

What Readers Want to Know

What did the study find? Sediment cores from Lago Cobá and Cenote Xcach on Mexico's Yucatán Peninsula show mercury reached toxic levels for centuries during the Preclassic and Classic Maya periods.

Where did the mercury come from? Mostly from cinnabar, a red mercury-sulfide mineral used as pigment. It was likely imported, probably from present-day Honduras.

How high were the levels? The most polluted samples reached 2.3 parts per million, more than twice a commonly used toxic effect threshold of 1 ppm for mercury in sediments.

Did mercury cause the Maya collapse? No. The lead author says it may have been a contributing factor, alongside drought, deforestation, and erosion.

Was methylmercury measured? No. Bacterial conversion to methylmercury is the likely pathway to harm, but the team did not directly measure it.

Why does this matter today? Mercury lingers in sediments for centuries, and modern sources still contaminate lakes. Check state fish advisories before eating locally caught fish.