Tree rings can reliably track mercury pollution in the atmosphere. Researchers studied a Yukon gold mining town that operated from 1905 to 1966. If you mercury to river gravel, it binds to gold, separating it from sediment. Heat the deposit. Mercury separates and you get gold. During heating, some mercury diffuses into the atmosphere, and trees absorb it. By examining cores taken from trees growing in the area, researchers got yearly accounts of atmospheric mercury levels...

What did they find?