WORKERS in yellow jumpsuits and respiratory equipment are stripping one end of a building down to its beams. The other end, shrouded in white plastic, must be vacuumed out and washed down before being exposed to air. The arsenic dust and white asbestos that have already been removed are temporarily stored in hundreds of hazardous-material bags surrounding the building.

Welcome to the Giant Mine, an abandoned gold mine in Yellowknife, capital of the Northwest Territories (see map). The Canadian government first took charge of it in 1999 after the owner declared bankruptcy and walked away. It is one of an estimated 10,000 orphaned or abandoned mines in Canada’s north that are now the government’s responsibility. And it is full of arsenic trioxide, a compound that is produced by heating arsenopyrite ore, a mineral that has traces of gold. Arsenic trioxide is odourless, tasteless, highly soluble—and lethal. An amount smaller than a pea is enough to kill. The Giant Mine has 237,000 tonnes of the stuff.

The Giant Mine opened in 1948. For the first few years arsenic trioxide went up the smokestack as vapour and came down in the surrounding area as dust. Dust-collecting “scrubbers” were added to capture some of the poisons after newspapers began warning people not to drink water made from melted snow. Once captured, the arsenic, which has the consistency of talcum powder, was blown into underground chambers. As long as the poisonous dust was in the permafrost layer, the thinking went, it would freeze solid and no longer pose a problem.

That might have worked had mine managers not later decided to dig a series of open pits to extract more gold. Now much closer to the surface, the permafrost has melted. With water leaching into and out of the mine it was only a matter of time before the arsenic threatened the waters of Great Slave Lake.