Some of the worst mining disasters do not happen in mines. They take place at dams. After minerals are extracted from mines, there are waste materials — including sand, rock and chemicals. They're known as "tailings" and are permanently stored in damsconstructed of earth, rock-fill or concrete. But the dams can fail — from chemical erosion caused by the tailings and from disasters like floods and earthquakes. And the toll can be devastating.

That is the message in a report by the United Nations Environmental Programme (UNEP) released in November. It is the first time the U.N. has documented the widespread destruction caused by these dam failures.

In 2014, for example, a Canadian dam built on a weak foundation broke, spilling billions of gallons of mine waste into a nearby creek. The torrent of waste and water washed away trees and contaminated nearby lakes. Lingering environmental effects from the Mount Polley Disaster, as it is known, are still felt to this day.

In 2015, a similar event occurred in Brazil. Another mine tailings dam failed, spilling — again — billions of gallons of waste. The spill turned into a rushing mud flow that inundated parts of a village, pulling cars downstream and destroying hundreds of homes. Nineteen people died as a result. An investigation found the dam's collapse was caused by flaws in its design and drainage.

These spills were not isolated incidents, according to the 70-page report, which documents mining waste failures from all over the world. It also offers recommendations for government and mining industry leaders to help prevent such disasters in the future. But it is unclear if its recommendations will lead to change.

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