When an estimated 184 million gallons (697 million liters) of industrial waste spilled into Hungary’s Marcal River in early October, arsenic and mercury threatened to taint water supplies and degrade rivers, both at the site and for hundreds of miles downstream. In some ways, Hungary’s toxic mud disaster was a wake-up call, shining a spotlight on potential water pollution hotspots around the globe. (Read more about the Hungary spill.)

Where might disaster strike next?

Only a tiny fraction of the ore miners exhume contains gold, copper, lead, zinc, or the other metals they’re after. The rest is waste, or tailings, full of large quantities of metals and minerals ranging from benign to very toxic. These fine-grained wastes are often held in