There are 237,000 metric tons of arsenic trioxide locked in the subterranean caverns of Giant Mine on the edge of Yellowknife, an unwanted byproduct from what was once one of the largest gold mines in Canada’s Northwest Territories. Consider that it only takes 140 milligrams of arsenic trioxide to kill a person; there’s enough of the poison here to kill 1.7 trillion people. The local indigenous people refer to the arsenic as a sleeping monster. Company and government officials hoped the arsenic would remain frozen underground forever. But mining operations and climate change caused the permafrost to melt, raising fears in the city of 20,000 people that toxic material could mix with the runoff and slither into the nearby waters of Great Slave Lake, the world’s 10th-largest freshwater body. From there, it could snake 1,000 miles along the Mackenzie River to the Beaufort Sea in the Arctic Ocean, poisoning the wildlife, the land and the water along its path.

The gold mine was one of the largest in Canada’s Northwest Territories, producing 7.6 million ounces of gold between 1948 and 2004—and leaving behind a toxic legacy.
But extracting minerals from the North carries high, and enduring, risks and costs. Current estimates put the cost of cleaning up Giant Mine at $3.2 billion, making it the most expensive mine remediation in Canadian history. The last owner, Royal Oak Mines, went bankrupt and left the bill to the government…

Canada’s government estimates there are roughly 24,000 contaminated sites across the country, which will cost 10 billion Canadian dollars—or $7.25 billion—to clean up. “Mining is a necessary evil. Fundamentally, it’s a license to pollute,” said David Livingstone, former chairman of the Giant Mine Oversight Board, an independent advisory body that monitors the Giant Mine cleanup. 

The arsenic at Giant Mine is the legacy of five decades of gold mining. Between 1948 and 2004, the mine produced 7.6 million ounces of gold, worth roughly $20 billion at today’s prices. There was so much gold that local indigenous people named Yellowknife “Somba K’e,” which means Money Place…But the precious metal was embedded in arsenopyrite, a mineral containing iron, sulfur and arsenic. To get to the gold, miners had to roast the rock, a process that also transformed the once-stable arsenic into toxic gas.

In the early years, miners ejected the arsenic out of a smokestack, believing the poison would be diluted in the air. Instead, the smoke condensed and fell to earth as a fine dust. It collected in the water and on the land. Cows and other livestock sickened and died. In 1951, an indigenous toddler died after eating arsenic-laden snow. After that, miners collected the dust and pumped it back underground, theorizing the arsenic would remain frozen in permafrost. For decades, the system worked. But the Canadian North is warming at a rate four times faster than the rest of the world, and the once-frozen ground is thawing.  Water is trickling into Giant Mine, a potentially catastrophic situation because the highly soluble arsenic trioxide could get carried into Baker Creek.
The creek runs through the mine site, and then into Yellowknife Bay in Great Slave Lake.
The headwaters for the Mackenzie River originate at Great Slave Lake. The river travels more than 1,000 miles to the Beaufort Sea in the Arctic Ocean…

Today, maintenance of the mine is an ongoing task. Waste materials, called tailings, are kept in large reservoirs around the mine. To stop dust contaminated with arsenic and cyanide from flying downwind, workers spray the tailings with a chemical mix called Rhino Snot, a blue-green dust suppressant developed by the U.S. military. But there are still times when wind carries dust southeast toward Ndilǫ, an indigenous community located on the west side of Great Slave Lake’s Yellowknife Bay, less than 2 miles from the mine as the crow flies. When the clouds of dust descend, the Yellowknives Dene council calls residents to warn them to shut their windows and stay indoors, said Ndilǫ Chief Fred Sangris, one of two chiefs of the Yellowknives Dene. “We tell them the mine is coming,” he said.

Sangris is so angry about what Giant Mine has done to the Dene’s traditional hunting grounds that he can’t even look at it. “It’s a poison place,” he said. “It’s a place to avoid.”…“They say forever, and they mean 100 years,” he said. “They don’t know what forever is.” 

Excerpt from Vipal Monga et al., Deep in an Abandoned Gold Mine, a Toxic Legacy Lurks, WSJ, May 5, 2025