As Nepal prepares for a new climbing season, the search for missing Sherpas on Everest continues.

Three Nepali Sherpas are missing after being buried by a block of snow on Mount Everest, according to a statement from Nepal’s Tourism Department. The trio were delivering materials to construct and clear a route to be used by climbers for this year’s spring climbing season. A search and rescue operation is being carried out using helicopters and teams on the ground. The avalanche took place between the base camp and Camp 1 at the place called the Dam. The area around the treacherous Khumbu icefall is one of the most treacherous sections of a route used to summit the 8,848.86 meter (about 29,032 feet) mountain.

Spring is the prime time to climb Mt. Everest, although some mountaineers may climb in the less favorable autumn season. Nepal is home to eight of the 10 highest mountains in the world, so mountaineering is a significant source of revenue, employing large teams of Sherpas, guides and porters to support groups of climbers.