In ancient and medieval times, the main source of lead came from melting galena, a lead-silver ore, to make silver coins. “When you try to extract silver, you also extract lead, something like 100,000 times as much lead as silver,” says McConnell. When the ore is boiled to separate the two metals, lead bubbles off first, escaping into the atmosphere. Then, from the start of the Industrial Revolution until the 1970s, lead entered the air primarily through fossil fuel burning; coal and oil contain the metal. At every point in history, winds transported this pollution to the Arctic, where it settled, becoming encapsulated in ice with each coming snow.