Eight climbers who were killed in the deadliest day in Mount Everest's history may have died after "sky fell in", according to a new study.

An analysis of weather patterns in May 1996 suggests the mountaineers died when the stratosphere sank to the level of the summit, nearly 9,000 metres (29,000ft) above sea level.

Normally the peak sits just below the atmospheric layer. But on May 10 1996, there were two fast-flowing air streams, called jet streaks, moving over the mountain. The freak weather caused pressure and oxygen levels to plunge within the "death zone" - the area above 8,000 metres where oxygen is extremely thin.

When the climbers set out, conditions seemed perfect. But 24 hours later, eight of them were dead.

The worst tragedy Everest has ever seen made headlines around the world, and was put down to a freak storm.

Now, a Canadian scientist has put forward an explanation of how it came about. Dr Kent Moore believes the climbers died when the sky literally "fell in" on them, depriving them of oxygen.

29,000 feet above sea level, the summit is just below the stratosphere. Uniquely on that day in May, there were 2 jet streaks - rivers of very high winds - moving directly over the mountain.

The theory is that they pushed the stratosphere boundary down onto the summit - as if the sky had fallen in. The ensuing thunderstorm caused oxygen levels to drop even further, leaving the climbers just 20 per cent to breathe. Simon Lowe was running a summit attempt on the northern side of Everest on that fateful May day - he turned back as the freak storm swept in. He accepts the science but says that climbers are already more aware of what can happen on the mountain