In the 50 years since Tenzing Norgay and Sir Edmund Hillary first scaled Mt. Everest, so much refuse piled up on the world’s highest mountain that some took to calling it the world’s highest garbage dump. By the early 1990s, an estimated 50 tons of metal, glass, and plastic, including many hundreds of abandoned oxygen canisters, had been left behind by climbers struggling to reach the summit. The Nepali government has made notable progress during the past decade in tackling the trash problem, slapping fines on climbers who don’t bring their oxygen bottles and other equipment down with them and organizing cleanup expeditions, some of which were funded by royalties paid by climbers. The main source of pollution now is human excrement left behind by climbers, said Ken Noguchi of Japan, who has led four cleanup missions on the mountain since 2000. Even as climbers become more conscientious about treading lightly, the high-altitude landscape continues to be threatened by an ever-growing number of expeditions that try to make it to the top each year.