What is the Khumbu icefall - and why has it left climbers trapped on Mount Everest?

Climbers attempt the Khumbu icefall in 1996 Credit: Reuters

An estimated 100 climbers and guides were left stranded on the side of Mount Everest when Saturday's earthquake rocked Nepal, triggering an avalanche which killed at least 17 others.

Clear and calm weather has allowed helicopters to begin the rescue mission this morning, bringing two people at a time back down the mountain - but dozens more are still trapped, and a number missing.

They are unable to take what would be the usual route back down from camps one and two, as the Khumbu icefall, treacherous even on a good day, has been rendered impassable by the avalanche.

The Khumbu icefall, pictured here last year, has been badly damaged by Saturday's avalanche Credit: Reuters

What is the Khumbu icefall?

The Khumbu icefall is a passage of constantly-falling ice from the head of the Khumbu glacier, at the point where the ice begins to melt. It is found at around 5,486 metres (17,999ft) above sea level on the Nepalese side of Mount Everest, not far from base camp.

It has been labelled the most dangerous part of the trip up the South Col route to the summit of the mountain.

What has trapped the climbers?

Saturday's avalanche, sparked by the 7.9 magnitude earthquake, struck between the icefall and base camp, leaving at least 17 dead and dozens more injured as well as flattening tents, burying equipment.

The route - while reportedly still there in essence - has been badly damaged by the avalanche and even more unstable than usual.

The icefall is not far from base camp Credit: Reuters

Why is the Khumbu icefall so dangerous?

The melting of the ice means the Khumbu glacier moves relatively quickly down the mountain - around thee to four feet every day.

This means large crevasses can open with little to no warning, while frozen towers can fall at any moment, sending large blocks of ice tumbling down onto the route.

For this reason, climbers usually brave the passage early in the morning, when the freezing overnight temperatures mean the ice is less prone to moving.

The route is fixed by ladders and ropes every year by experienced climbing guides, to ease the way for the hundreds of tourists and climbers who want to try to reach Everest's summit each year.

An avalanche last year killed 16 sherpas who were preparing the icefall for the spring climbing season.

What can be done to search for those missing?

Search and rescue expert James Perry told ITV's Good Morning that a number of methods can be used to try to locate anyone trapped beneath the snow.

Among the potential courses of action which could be taken by rescue teams is the use of sniffer dogs, carbon dioxide detectors - to try to determine if anyone is breathing underneath the surface of the snow - and line searches, which involve people lowering rods into the layers of snow to try to find any bodies trapped.