Get the Popular Science daily newsletter💡 Breakthroughs, discoveries, and DIY tips sent six days a week. A trio of ancient reservoirs in present-day Guatemala is revealing both the strength—and limitations—of Mayan water science. While the civilization’s purification techniques resulted in comparatively clean drinking sources, archaeologists say the unknowable consequences of a commonly used , deep-red pigment consistently subjected the Indigenous population to toxic mercury poisoning. City of three reservoirs It’s no accident that Mayan society lasted thousands of years. The pre-industrial Central American civilization boasted sprawling, densely populated urban centers, complex agricultural practices, and effective civic planning structures. But above all else, access to reliably potable water was an absolute necessity for these cities to flourish. For example, the city of Ucanal at the mouth of the Belize River in northern Guatemala likely featured anywhere from 8,000 to 11,000 inhabitants. This required maintaining at least three nearby reservoirs, each for different uses.  Between 2018 and 2024, researchers from the University of Montreal in Canada explored the archaeological remains around Ucanal’s reservoirs. The results, detailed in …