An avalanche is an uncontrollable flow or cascade of large masses of snow, ice, rock, mud and other debris down a mountain. There are many reasons why avalanches occur, such as when a constantly accumulating mass of debris can no longer be contained by a mountain slope, or when an earthquake ruptures a mountain or cliffside, or when a slab of ice and snow is released by strong weather.

Throughout recorded history, avalanches have become part of many natural disasters such as earthquakes and volcanic eruptions, resulting in a considerable loss of life along with tremendous damage to communities and personal property. Enumerated below are eight of the worst recorded avalanches over the last 500 years ranked according to the number of human casualties:

Yungay, Peru, 31 May 1970 (20,000 casualties) - A huge earthquake shook northern Peru, unleashing millions of tons of snow and ice from the slopes of South America's second highest mountain, 22,000-foot-high Nevado de Huascarán, overlooking the city of Yungay. The rampaging ice caused lakes and reservoirs to overflow, sending down debris at a rate of three miles per minute. The entire city was buried beneath 100 million cubic yards of debris. This is considered as the worst avalanche in history.

Tyrolean Alps, Italy and Austria, 13 December 1916 (10,000 casualties) - In a 24-hour period, Italian and Austrian forces died in a series of avalanches caused by heavy snowing. An entire barracks was destroyed, burying 250 officers. Observing the catastrophe, troops on both sides decided to fight each other by creating man-made avalanches. Up to 1918, mountainsides were bombed, resulting in an additional 40,000 deaths.

Ranrahirca, Peru, 10 January 1962 (3,500 casualties) - Tons of melting snow from a storm a day earlier flowed down Nevado de Huascarán and fall into a canyon 3,000 feet below, obliterating two villages and killing almost a thousand people. Strong winds carry the melted mass along the narrow walls of the canyon to other communities which are instantly wiped out, too. Finally, a catastrophic 39 million feet of snow and debris exited out of the canyon and burst into the the town of Ranrahirca, burying everything in sight.

Plurs, Switzerland, 4 September 1618 (2,427 casualties) - A snowslide, known as the Rodi avalanche, buried the town of Plurs. Only four people, who were away from the village at the time, survived.

Swiss-Austrian-Italian Alps, 1950-1951 (over 265 casualties) - In an unusual winter season known as the Winter of Terror, heavy snowstorms dump huge masses of snow and ice, creating a series of over 600 avalanches that occurred throughout the different national boundaries encompassed by the Alps. In Austria, more than 100 lives were lost, while Switzerland lost 92 lives.

Blons, Austria, 12 January 1954 (over 200 casualties) - In what is regarded as Austria's worst avalanche, the village center of Blons was destroyed by a mid-morning dry-snow avalanche called the Falv. Nine hours later, a second avalanche known as the Montclav moves 3,800 feet downhill in less than a minute, wiping out the village and burying 115 people, which included a team of rescuers.

Lahaul Valley, India, early March 1979 (200 casualties) - A series of snowstorms over a five-day period results in avalanches that cascade down the foothills of the Himalayas in Himachal Pradesh state, dumping up to 20 feet of snow on the inhabited valley.

North Ossetia, Russia, 21 September 2002 (150 casualties) - A collapsed glacial chunk on Mount Kazbek turned into a 20-million-ton avalanche that rampaged down Karmadon Gorge, burying several villages. Among the casualties was Russian actor/director Sergei Bodrov Jr., and his production crew, who were on location for a film shoot.