Two catastrophic mine dam failures, one in Canada and one in Brazil, made worldwide headlines in recent years for the downstream damage they caused.
PolyMet officials say their tailings basin and dams will be so different from those that failed that any comparison is unfair and meaningless.
But PolyMet's critics say the failures show the inherent danger of so-called modern mining operations, especially tailing basin dams, even under regulated conditions.
Canada
On Aug, 4, 2014, a dam holding back mine waste at the Imperial Metals' Mount Polley copper and gold mine in British Columbia failed, releasing millions of gallons of water mixed with mine waste - 26 million cubic yards - that flowed downstream and into Polley Lake, then on to Quesnel Lake and Cariboo River. The tailings basin was emptied within four days.
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Tests downstream found elevated levels of selenium, arsenic and other metals.
An independent, government-ordered panel of experts concluded the cause was an inadequately designed dam that didn't account for drainage and erosion failures beneath the basin. The Mount Polley basin design called for a rock buttress to be placed against the perimeter embankment to strengthen that embankment so as to meet the required factor of safety. But that never happened. About two months before the break, the basin, which wasn't big enough to store major rainfall events, was overtopped by excess water, further weakening the dam embankment soils. The ultimate cause of the Mount Polley Dam Failure was determined to be an unstable clay layer below the embankment. But the independent review concluded that the dam would have held if had had been designed properly and had the rock buttress been added.
Imperial Metals had a history of operating the tailings basin beyond capacity since at least 2011.
The dam has since been rebuilt and operations at the mine have resumed at a lesser scale.
The Mount Polley tailings dam was 131 feet tall from the base to the top.
Brazil
On Nov. 5, 2015, the Fundao tailings basin dam burst at the Samarco iron ore mine and processing center in Minas Gerais, Brazil. The flood of mineral waste - some 42 million cubic yards of water and mine waste - surged through the River Doce valley. Trees, roads and bridges were swept away and nearly 5,000 acres of farmland was swamped. Nineteen people were killed and more than 700 left homeless. Nearly 5,000 people worked at the mine, co-owned by iron ore giants BHP Billiton and Vale.
The company said the deluge wasn't necessarily toxic, but the sheer volume of solids killed fish for miles downstream, and hundreds of fishermen have sued for damages.
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A report into the cause of the dam failure by New York law firm Cleary Gottlieb found both construction and design flaws. It also blamed tweaks to the design for water seeping into the infrastructure and "liquefying" the dam's base, causing soil in the wall to weaken and become structurally deficient.
The investigation also found that a change in the dam's design resulted in less-efficient drainage. And a number of small earthquakes on the day also exacerbated the collapse.
The report stopped short of assigning blame for the accident.
BHP, Vale and Samarco struck a deal with the Brazilian government to pay at least $6.5 billion to fund the Renova Foundation to help restore the environment. But the deal has been suspended after a separate, $50 billion civil suit was lodged by the Federal Public Prosecution Service.
Work has begun to repair and rebuild the dam, but it's unclear if or when the mining and processing operations might resume. The owners have said iron ore pellet production at the plant will be reduced from 30 million tons annually to about 19 million tons when the mine does restart.
The dam was 574 feet tall.
