On this day in 2014, 16 Nepali mountaineering guides, most of them ethnic Sherpas, are killed by an avalanche on Mt. Everest, the Earth’s highest mountain. It was the single deadliest accident in the history of the Himalayan peak, which rises more than 29,000 feet above sea level and lies across the border between Nepal and China.

The avalanche, which occurred around 6:30 a.m., swept over the Sherpas in a notoriously treacherous area of Everest known as the Khumbu Icefall, at approximately 19,000 feet. At the time, the Sherpas had been hauling loads of gear for commercial expedition groups. The disaster, in which no foreigners were killed, reopened debates about the dangerous risks undertaken by Sherpas for their typically affluent clients (in addition to lugging most of the supplies for an expedition, Sherpas are responsible for such tasks as setting lines of fixed ropes and ladders for climbers), as well as the over-commercialization of Everest, where human traffic jams during the spring mountaineering season and massive amounts of litter have become common.

In 1953, New Zealander Edmund Hillary and Sherpa Tenzing Norgay became the first people to officially reach the summit of Everest, which the British named in 1865 for George Everest, a Welsh-born surveyor general of India. Andrew Waugh, his successor as surveyor general, chose the mountain’s moniker; it’s unlikely George Everest ever saw the peak named in his honor. (Meanwhile, the Nepalese refer to the mountain as Sagarmatha, while Tibetans call it Chomolungma and the Chinese know it as Zhumulangma Feng.) Since Hillary and Norgay’s historic achievement, more than 4,000 people have scaled Everest, while at least several hundred others have perished in the process. In 1996, eight climbers were caught in a storm on the mountain and died, as chronicled in the best-selling book “Into Thin Air” by Jon Krakauer. That season, a total of 15 people lost their lives on Everest, making it the deadliest season until 2014.

In the aftermath of the April 18th tragedy, a number of Sherpas boycotted the remainder of the climbing season, out of respect for the 16 guides who were killed and also to protest such issues as the pay and treatment of Sherpas. As a result, many commercial expedition companies opted to cancel their planned ascents.