On the slopes below the Altiplano are highlands with fertile soils and a warm, wet climate. People fell trees to make fields where they grow maize and vegetables to keep a family alive - just. It should be rich farming land, but there is a big problem. Tree felling and ploughing loosen the soil, and when it rains - as it often does in Bolivia - soil is washed away down the slopes. Gullies form, and the mountain rock is left bare: useless for growing food. Much of the land is no longer fertile: soil erosion has taken place.

The poverty that this causes forces people to look for other ways of making a living. For hundreds of years, native Bolivians have mined metals in these hillsides, but others took the riches: first the Incas, and then the Spaniards.

In the last few years there has been a gold rush in South America, with big mining companies from the United States and Canada coming to exploit the land for the precious metal. For most ordinary folk, escaping from poverty is only a dream - but a dream they pursue, at great danger to themselves.

Gold mining involves sifting vast amounts of muddy sediment from streams, through mercury, which separates it. The mercury is then vaporised into the air. Mercury is a lethal poison. It can kill you if you breathe or swallow it. The vapour falls onto the people and the land, and the mercury waste pours into rivers which flow into the Amazon. Many Amazonian Indians, like the Kayapo, now have dangerous amounts of poisonous mercury in their blood.

Avalanches

When avalanches happen depends upon slope angles, vibration, changes in temperature, and the weather. But nobody really knows exactly when they're going to happen - we can only guess.

The big problem is that avalanches are often set off by the skiers themselves - especially if they don't keep to special marked ski runs, but go off across seemingly safe untouched snow.

People trapped in avalanches stand little chance of survival. The snow compresses, and although most people are near the surface, they are trapped, unable to move. If they're not rescued within 15 minutes, most die of asphyxiation or hypothermia.

If you want to know how likely an avalanche is to happen, you can input today's weather into a computer, which tells you if avalanches tend to happen in similar weather conditions. Despite only working on statistical correlation, the predictions seem to work well enough to be used throughout the world.

More people went skiing in the Alps last year than ever before. But because rescuing them is now more efficient, fewer people than usual were killed. Avalanches will go on killing people as they have done for thousands of years. It may have been an avalanche which killed the oldest human body ever found intact, the 'Otzi ice-man', who lay in the ice for 53 centuries on the high mountain border of Italy and Austria.