MARIANA, Brazil—Half an hour’s drive from this colonial town in southeast Brazil, trees suddenly give way to what looks like a desert salt flat. It is a 2-mile-wide valley filled with mine waste.

On Nov. 5, an earthen dam holding back this sea of sludge collapsed, releasing a deluge that killed 19 people, destroyed villages and traveled more than 400 miles to the Atlantic Ocean, where it left a reddish-brown plume visible from space. As tall as a 30-story building and holding enough refuse to fill 19 Dallas Cowboys stadiums,...