Mercury, also known as quick-silver, is the only metal that's liquid at room temperatures. It's very useful for thermometers, barometers, and dental fillings. Because mercury forms an amalgam with gold, it was used in mining. Some of that mercury from old mines erodes into streams, rivers and lakes. And some of it vaporizes into the atmosphere.
Newspapers carry warnings about eating certain kinds of fish, because of their mercury content.
The form of mercury in fish is especially toxic to humans. It's in an organic form called methyl mercury. The methyl form is produced in environments rich in organic materials. Anaerobic bacteria, who thrive without oxygen in sediments, scavenge elemental mercury and excrete it as methyl mercury. Some plants also scavenge mercury.