Why did the Maya flee the majestic city-state of Tikal? Experts say overpopulation, overexploitation of land, and droughts drove them out in the ninth century AD. Now, another cause: poisoned water. A new study says there's evidence of mercury and toxic algae ruining the drinking water just as residents were battling the dry season in what is now northern Guatemala, the Smithsonian reports. The research hinges on analysis of reservoirs, which—per sediment samples from the mid-800s—appear highly polluted. One source is cinnabar, a red mineral pigment used to paint Tikal's palaces and temples. Sadly, it's also mercury sulfide, which poisoned the water supply when it came off walls and flowed into reservoirs.
The reservoirs also turned up DNA traces of blue-green algae, which can create deadly toxins. What's more, the study found high levels of phosphates—likely accrued from centuries of cooking fires, food waste, and feces—which would have added nutrients that spurred massive blue-green algae blooms, per Ars Technica. In a twist, this only poisoned rich people's reservoirs when mercury flowed off the fancy, cinnabar-coated plazas. But that might have accelerated Tikal's decline if Mayans questioned whether their rulers had failed to satisfy the gods. "These events ... must have resulted in a demoralized populace who, in the face of dwindling water and food supplies, became more willing to abandon their homes," the study says. (Read more Mayans stories.)