Aluminum is prized for making products from kitchen foil and beverage cans to Teslas and aircraft. But the silvery metal—abundant, cheap, lightweight, and corrosion-resistant—has a dark side: red mud. This brownish red slurry, a caustic mishmash of metal- and silicon-rich oxides, often with a dash of rare earths and radioactive elements, is what’s left after aluminum is extracted from ore. And it’s piling up. Globally, some 3 billion tons of red mud are now stored in massive waste ponds or dried mounds, making it one of the most abundant industrial waste streams on the planet. That’s led to environmental and human disasters, such as 10 people in Hungary who died in 2010 when a dam holding back a massive red mud pond gave way. Now, researchers are working to lessen these dangers by finding ways to recycle and reuse red mud. Some researchers are developing ways to extract the valuable rare earth metals, while others turn the mud into cement or bricks.