The Yankee Doodle Tailings Pond in Butte, Montana, is used as a dumping site for leftover materials from nearby copper mines. It covers an area of roughly 2.5 square miles (6.5 sq. km) and is contained by a 650-foot (200 m) tall earthen dam — one of the largest of its kind in the United States. To help offset the acidity of mine waste, lime rock is added to create a non-acidic tailings slurry.46°02'37.3"N, 112°30'23.2"W