A few days of warm weather had previously melted the surface, which had turned to ice. The new snowfall couldn't bond to this hard icy layer. No one knows exactly what triggered it, but in seconds, over a third of a million tons of snow tore down the mountain at a devastating mph. This monster wiped out everything in its path and the town of Galtur was crushed under its weight. people died. It was the worst disaster for years. Here in the mountains, the weather has one more trick. The snow that brings the devastation of avalanches also creates cold's most enduring monument, a glacier. We've to go along the white ice to the green ice up here, the glacier... a long way still. Mountain guide Russel Brice is going to take me to the source of the Argentier, one of the most impressive glaciers in the Alps. It's quite delicate, isn't it? Climbing! Remove slack! Sorry. Yeah, that's good. It seems impossible to get a grip. Keep me tight. You can get your axe in there. Ah! OK? How you doing? All right. Just to the left, there. Yeah. Oh, what a relief. Yeah. Excellent. That's good. Not very graceful. Every metres up, the temperature drops by half a degree. It's the mix between the cold air up here and the warm air blown into the mountains from below that can make the weather so ferocious. It's also what creates the snow which makes the glacier. Finally, the peak of the mountain, where it all begins. Here we are, the old Holy Grail. See here? This is the collection area. This is what collects all the ice and snow, where a glacier would actually start. This is truly the most spectacular sight I've seen in all my work on this programme. It's also one of the coldest and most terrifying sights. Those crevasses, how big and deep are they? Maybe metres deep. metres deep! If somebody's down there? Not a chance, you die. To understand how it forms and what it does to the weather, we have to climb it. Down here at the base, it seemed like a good idea! The glacier has been here for over , years. It takes the ice years to travel from the top to down here. Way above us lies the source of this stream of ice. Surprisingly, it starts its life as snow. Over time, the crystals bond together to form the ice. As it falls, year after year, the layers of snow become crushed into little ice crystals that fuse together. The weight of the new snow compresses the ice below even more, eventually turning it to pure blue, glacier ice. When the glacier is big enough, it starts to move and, over thousands of years, a vast river of ice is formed. As it flows, it breaks into enormous crevasses. The glacier is over six miles long and two miles wide. It's amazing in here. It's going to be quite hard to get out. Look how hard the ice is. Extraordinary. It will shatter. See how it's cracked all round here. How do you get a grip? It's what we have to do. What is it that makes this so smooth with all these layers? The sheer weight of the ice on top of itself. If you take a handful of snow and squeeze it and squeeze it really hard, you'll make ice. The amount of pressure that created that, the pressure! How are we going to climb that? Quite hard ice here.