Hilaree Nelson and her partner James Morrison, the world’s most prolific ski mountaineers, braved several mountains in the world, always pushing their limits.

At noon on Monday, they were on yet another of their mountaineering adventures when tragedy struck.

Hilaree and James were returning together after the successful “true summit” of Mt Manaslu, the eighth-highest mountain in the world at 8,163 metres.

“Something unusual happened above 8,000 metres, the death zone, where the pressure of oxygen is insufficient to sustain human life,” Jeevan Ghimire, managing director of Shangrila-Nepal Treks, the company handling their expedition, told the Post.

The mountaineering duo had planned to ski down the sharp knife ridge in a quest for one of the most dangerous vertical drops in the world. The planned drop was 8,000 metres down its face.

“Unfortunately, Hilaree separated and fell into a crevasse,” said Ghimire. “We don’t know the exact reason behind it but it was unimaginable. It is difficult to believe. Let’s hope she is fine.”

Bigyan Koirala, an official at the Department of Tourism, the government agency that issues the climbing permits, said a helicopter is on standby for a search and rescue early in the morning on Tuesday.

Hilaree, the 49-year-old mother of two, was the first female to link two 8,000-metre peaks, Everest and Lhotse, in one 24-hour push in May 2012.

In 2018, she returned to Lhotse, the fourth highest mountain in the world at 8,516 metres, to ski from the summit, one of the most prized un-skied lines in the world.

In 2018, she was named captain of The North Face Athlete Team, a title only one other athlete has held: Conrad Anker.

As a role model to other women adventurers, Hilaree continues to push the envelope of not only geographical exploration but also an exploration of our human capacity to challenge social norms, The North Face, an American outdoor recreation products company, said on its website.

In a separate event on Monday, a powerful avalanche occurred between Camps III and IV of Manaslu, killing Anup Rai, a mountain guide, police said. Several others were injured.

“Four climbing guides have sustained critical injuries,” said Koirala, the official of the tourism department. “Nine others had minor injuries.”

The search and rescue team was mobilised immediately after the incident.

“We have airlifted three people and admitted them to Karuna Hospital in Bansbari,” said Shree Hari Kuikel of Kailash Helicopter Services.

The high-altitude workers were transporting goods to Camp IV.

Icefall doctor Yukta Gurung, who is at the base camp, said that the avalanche was triggered by continuous snowfall. “It had snowed unceasingly for 15 days. The area was covered in at least five to six feet of snow; the piled-up snow ultimately gave way, triggering the avalanche,” he said.

Last year, mountaineers scaled the main summit of Mt Manaslu in autumn for the first time since 1976.

Led by Mingma Sherpa, better known as Mingma G, 14 Nepali and eight foreign climbers achieved the rare feat on September 27 for the first time in 45 years. The main peak is 8,163 metres high, and the mountaineering fraternity has called it the “true summit”.

Many foreigners had been debating if Manaslu’s main summit could be climbed in autumn.

Mt Manaslu has been climbed every year, and even reaching the fore-summit, which is just below the main summit, is accepted as a successful ascent. The main summit is 6-7 metres higher than the fore-summit, and it had not been climbed since 1976.

This year, a record 404 permits have been issued to climb Mt Manaslu.

(Hariram Upreti from Gorkha contributed reporting.)