Federal officials are scrambling to clean up a crumbling, abandoned northern gold mine that is in imminent danger of releasing massive amounts of arsenic, asbestos and other toxins.

"It's pretty scary stuff," said Mark Palmer, senior adviser on Aboriginal Affairs and Northern Development's Giant Mine Project, which describes a proposed cleanup of collapsing, poison-filled buildings and caverns on the shore of Great Slave Lake as an emergency response.

"We are worried they are going to fall down and if that happens there will be a release."

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The Giant Mine just outside Yellowknife was an economic mainstay for 50 years. But its gold was locked within crystals of arsenopyrite, and after the mine finally closed in 2004, about 237,000 tonnes of highly toxic, water-soluble arsenic trioxide remained on the site.

Most of the arsenic was blown back underground, where huge dustpiles of it sit in 15 subterranean chambers, some big enough to swallow an 11-storey building. About 3,600 cubic metres of arsenic and arsenic-contaminated material remain in surface structures – uncontained and, in many cases, exposed to the elements.

A $488-million plan to clean up the site and freeze the underground arsenic in place – Canada's biggest environmental cleanup – is before northern environmental regulators. But engineering reports say the buildings and some of the underground caverns have deteriorated so badly that the cleanup must begin as soon as possible to contain various poisons, which also include mercury, cyanide and PCBs.

"Many of the underground elements are showing signs of failure, including the formation of a sinkhole at the surface," says the department's application for the emergency cleanup.

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"There is the potential for significant impacts to the environment and injury to humans through falling cladding, partial building collapse and arsenic and asbestos exposure to humans and wildlife."

Arsenic poisoning starts with headaches, confusion, severe diarrhea and drowsiness. When the poisoning becomes acute, symptoms may include diarrhea, vomiting, blood in the urine, cramping muscles, hair loss, stomach pain and more convulsions.

Continued poisoning leads to a coma followed by death.

Photographs from the site show piles of arsenic dust lying exposed inside the old flues that used to carry it underground. Those flues are pulling away from the building they were attached to and slowly collapsing as concrete and wood pillars that held them up rot away.

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Snow blows freely through derelict walls and roofs onto arsenic-contaminated equipment. Asbestos insulation waves in the wind as it flakes off old pipes and buildings.

Underground, arsenic-stuffed caverns are in danger of falling in from the surface or collapsing into mined-out areas below. The mine's main smokestack is crumbling.

Palmer said the work needs to start this summer before the poison starts escaping.

"We're saying it could happen. It's got to be an unacceptable risk.

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"Our main goal is the health and safety of northerners and the people on the site."

He said he hopes regulatory approvals will be in place by June.

The cleanup would be dicey. The worst-contaminated surface building, where arsenic and gold were separated, would have to be sealed off as it was taken apart. Workers would have to wear full hazmat suits and breathe supplied air.

As much of the arsenic as possible would have to be cleaned up and sent underground. Solid waste from buildings would be removed and stored until full remediation began.

Palmer estimates an emergency cleanup would take two summers, by which time a full cleanup plan would likely be through the regulatory process. He was unable to supply a cost estimate, as the project is just going out for bids.

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Whatever the cost, it would be borne by taxpayers. Giant was built before northern miners were required to post environmental cleanup bonds — in fact, the mess left at Giant is one of the reasons such legislation was drafted.

The cost of addressing Giant's imminent toxic threat is just the latest addition to the mine's bitter legacy.

Although it once provided a livelihood to generations of Yellowknifers, Giant was also the scene of a violent 1992 labour dispute that saw nine miners murdered in a deliberately set underground explosion.

After that, the failure of owner Royal Oak Mines left a pension fund badly depleted by management's purchase of company stock. Many longtime employees got a fraction of what they were owed.

The widows of the murdered miners were also forced to go through years of civil litigation in an attempt to win a $10-million wrongful death settlement, which eventually failed before the Supreme Court.

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Striking miner Roger Warren, convicted of the killings, remains in prison on a life sentence.