A series of USGS interactive maps have recently been published. The maps identify locations and levels of mercury contamination resulting from historical gold mining California’s Sierra Nevada. … Many may not know that mercury was used by miners during the California Gold Rush to extract gold. Using a process called amalgamation miners would combine mercury with gold ore to form a gold-mercury amalgam. The mercury was then removed by heating. After the mercury had evaporated, pure gold was left behind. However, this method had severe environmental consequences.