Three Sherpa climbers were missing Wednesday after they fell into a deep crevasse on a treacherous section of Mount Everest just above the base camp, a Nepalese mountaineering official said.

They fell into the crevasse, thought to be about 160 feet deep, on Wednesday morning as they were moving toward the first camp on the world’s highest mountain, said Yubraj Khatiwada of Nepal's Department of Mountaineering.

A rescue helicopter was trying to locate them while rescuers searched on foot, he said.

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The area is the Khumbu Icefall, a constantly shifting glacier with deep crevasses and huge overhanging ice that can be as big as 10-story buildings. It is considered one of the most difficult and tricky sections of the climb to the peak.

In 2014, a chunk of the glacier sheared away from the mountain, setting off an avalanche of ice that killed 16 Sherpa guides as they carried clients’ equipment up the mountain. It was one of the deadliest disasters in Everest climbing history.

Hundreds of foreign climbers and about the same number of Nepalese guides and helpers are expected to attempt to scale the 29,032-foot mountain during the main climbing season that began in March and ends at the end of May.

Climbers have begun to settle in at the base camp to acclimatize to the weather and altitude while the Sherpas place ladders and ropes and carry supplies to the upper camps for their clients.

The Sherpas also set up tents stocked with supplies and oxygen for the foreign climbers.