Landslides and avalanches are catastrophic shifts of rocks or snow, which wreak havoc wherever they happen.

The steep valleys of South Wales are rich in coal. Villages grew up beside the coal-mines, and for most of this century the region's main industry was digging out the 'black gold'. Waste from mining was piled up on vast tips called 'spoil heaps', which grew and grew. Nobody really bothered about how or where the tips were piled.

One spoil heap, 'Tip No. 7', was 100 metres above the mining village of Aberfan in the South Wales coalfield. On the morning of 21 October 1966, it was raining as the children of Pantglas Junior School gathered for assembly. There was a terrible rumble as Tip No. 7 suddenly slipped and engulfed the school.

There had been no warning! Thousands of rescuers toiled all that day and into the night. Only one classroom survived. Only 29 children lived. 116 children and 28 adults died.

In the Andes of South America, Bolivia is a country with Amazonian jungle, semi-desert and the heavily populated Altiplano, a plateau 3,500 metres above sea level.

There is much poverty in the region, and many people have moved away from farming to find work in the metal ore mines on the hillsides - especially gold mines. Thousands of Indians, farmers and settlers have started digging in their own open-cast gold mines.

Like farming, gold-mining is eroding the land. The pits are dug without any planning, and when it rains, water seeps into the ground and there's nothing to stop steep hillsides above from collapsing.

On 8 December 1992 at Llipi, 90 kilometres north of La Paz, the capital of Bolivia, it was raining heavily. Suddenly, hundreds of thousands of tonnes of hillside came crashing down on a do-it-yourself gold mine - and the miners were camping beneath it. Of the 1,200 people living around the gold mine, few survived.

The Alps stretch across Italy, France, Austria, Germany and Switzerland. Here not only land can come sliding down, but also snow and ice. Every year, 100 million holidaymakers head for the mountains. But some skiers never come home: around 150 people a year are killed.

The reason is avalanches: huge dislocations of ice and snow that can travel at 300 kph, weigh millions of tonnes, and impact with enough pressure to destroy the strongest buildings. In 1994, a team of British doctors who were going skiing encountered an avalanche.

Snow avalanches are the most common: thousands happen every year. There are two main types of avalanche: 'slab' and 'loose'. Slab avalanches happen when one layer of snow slides off another because there is ice or water in between, and nothing to prevent the fracture. Loose avalanches happen when snow breaks away forming tumbling clouds.

Forests can slow avalanches; but forests are disappearing. Fences and walls can also slow avalanches down, and big concrete wedges can split the flow to protect buildings and people.

Sometimes the best form of defence is attack: gun shot and explosives are fired to start avalanches intentionally and so reduce the chances of people being trapped by accident.

Avalanches are not predictable, but attempts at predicting the unpredictable are getting more successful. Now computers are starting to help. One system has a database of avalanches and what the weather was like when they happened.