Copper’s allure has endured for millennia. Both ancient and modern mines for the extremely useful metal abound in North America’s Lake Superior region; long before modern miners extracted the ore from deep underground, local Indigenous communities dug it from shallow pit mines.

These prehistoric mines’ ages were a “long-standing mystery,” says David Pompeani, a geologist at the University of Hawaii at Mānoa. Previous research used archaeological remnants to evaluate when mine sites were active, but later mining at the same sites often obliterated ancient artifacts, Pompeani says. To work around this, he and his colleagues took a different approach: instead of artifacts, they looked for signs of mining preserved in the environment.

For a recent study in Anthropocene, the researchers examined sediments from two small inland lakes near ancient mines on Lake Superior’s isolated Isle Royale in Michigan. Such sediments are affected by annual changes and thus act a little like tree rings. Each layer is a snapshot of what happened in a given year, including weather events, wildfires—and pollution.

Even preindustrial copper mining produced pollution, mainly from lead impurities in copper deposits. “Lead is a good proxy to record human impact … it’s not a metal we can get in a natural way,” says Francisca Martínez-Ruiz, a geochemist at Instituto Andaluz de Ciencias de la Tierra in Spain, who was not involved in the study.

Before modern machinery, extracting copper was labor-intensive. Native Americans hammered it out of the rock—hard, dusty work that lofted fine particles of stone and metals into the air. Pompeani says they probably also used bonfires to warm this rock, softening the copper and liquefying the easily meltable lead. These fires volatilized the lead and wafted it over the surrounding area, sprinkling particles onto the land and lakes. Analyzing lake sediments, the researchers found evidence of a peak in lead pollution around 6,000 years ago during the Archaic period. This suggested a simultaneous peak in large-scale copper mining—and matched archaeological evidence from the same period.

“The paper shows that the lead is a reliable proxy that can be used for reconstruction in this region,” Martínez-Ruiz says. She adds that similar studies of small lakes’ environmental pollution could be used to investigate human impacts worldwide.

Pompeani says the study confirms some of the world’s earliest-known large-scale mining efforts and puts a new spin on how Indigenous societies operated. “There’s this idea that hunter-gatherers couldn’t organize to conduct a mining operation,” he says. “Yet that lake sediment indicates that during the Archaic period, they mined to such a degree that we can detect it in the environment.”