Barcelona, Spain, has been experimenting with an unusual way to reduce pollution: buses powered by human waste. The V3 bus line runs on biomethane, a gas produced from sewage sludge from the city’s 1.5 million residents. That might sound gross, but the fuel is actually clean and odorless; in fact, engineers had to add an artificial smell to it so that drivers can detect leaks. Biomethane is more sustainable than traditional gas, and switching to it helped cut the bus line's greenhouse gas emissions—which scientists say contribute to climate change—by 80 percent. Other European cities, including London and Stockholm, have piloted programs that use sewage and household waste to power buses as well. But Barcelona’s approach also uses renewable electricity, which means it wastes even fewer resources. Transit officials hope biomethane can one day replace natural gas in long-distance buses too. “It’s a great idea,” passenger Leire Muños told French paper Le Monde. “If our excrement is useful for something, all the better!”