It is commons practice to dump mine tailings in an area near a mine. In areas where it rains, water may accumulate behind burms and the tailings which form an earthen dam. In valleys, lakes can form behind dams leading to potential failures. http://abcnews.go.com/International...village-dam-burst-mudslides/story?id=35024425 Scores missing as deadly mudslide flattens Brazil village http://news.yahoo.com/least-17-dead-dam-bursts-brazil-mining-waste-022224741.html http://news.yahoo.com/photos/two-da...-at-brazil-iron-mine-photo-1446841117332.html In Brazil, two dams failed flooding several villages down the valley. Dozens missing in Brazil mine disaster, death toll uncertain http://news.yahoo.com/dam-burst-vale-bhp-mine-devastates-brazilian-town-013112268--finance.html The water and mud was reported to be 15 to 20 in depth, or height. About 80% of the village Bento Rodrigues (which is about 93 miles (150 km) southeast of Belo Horizonte, Brazil's third largest city and the capital of the mining state of Minas Gerais) was destroyed. And authorities have trouble reaching the area.