An avalanche is one of nature's most terrifying and destructive forces. When a great torrent of snow and ice roars down a mountain at more than 200 miles an hour, there is no power on Earth to prevent it happening. Two or three miles long, at least a mile wide, sometimes up to 300 feet high and weighing millions of tons, an avalanche simply overwhelms everything in its path. From the Alps to the Himalayas and from the Rockies to the Andes, avalanches strike without warning, leaving the unwary no time to escape. Over time, avalanches have resulted in thousands of deaths, wiped out towns and villages, and devastated the landscape. More avalanche fatalities occur in the Alps (Europe's mightiest mountain chain) than anywhere else in the world. And around 75 percent of these occur on Mont Blanc, which, at 15 744 feet high, is not only the tallest mountain in the Alps, it is also one of the most unforgiving. This program looks at some of the world's worst-ever avalanche disasters. It describes what caused them in the first place, the havoc they created, and how the rescue and emergency teams worked together to save as many lives as possible. Years of painstaking research means that experts can now determine whether an avalanche in a particular region i imminent. But what they cannot do - and probably never will - is predict exactly when it will occur.