Kublai Khan and his imperial Mongol brethren were legendary warriors, masters of the Silk Road, and, according to a new study, terrible polluters due to silver mining. Geologists discovered this legacy by visiting Erhai Lake (pictured above) in the southwestern Chinese province of Yunnan. The team extracted 2.5-meter-long sediment cores that accounted for the last 4500 years of the lakebed’s history. They scanned the silt for heavy metal pollutants, namely copper, lead, silver, cadmium, and zinc. They noted a bump in copper contaminants around 1500 B.C.E., which corresponds with the start of China’s Bronze Age and the broader adoption of metal mining. But mining pollution remained low and relatively stable for the next two-and-a-half millennia—until the Mongols conquered China in the late 1200s C.E. The imperialists loved using silver—for coins, jewelry, art, and taxes—but the wood-fire smelting process released ash clouds filled with metal impurities like lead oxide. These plumes settle onto the earth or bodies of water. Lead, for instance, spiked in Erhai Lake, reaching a peak of 119 micrograms per gram of sediment by 1300 C.E. Heavy metal pollution during the Mongol (Yuan) dynasty was three to four times higher than modern industrialized mining, the authors reported online this month in Environmental Science & Technology. Although preindustrial pollution has been detected in lake sediments (and ice cores) around the globe, only a handful of studies have seen levels that exceed modern day—and this observation is the first from China. Lead in sediments can impact aquatic organisms for centuries, so the environmental consequences for Erhai Lake likely persist to this day.