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It is one of nature's most awe-inspiring and dangerous threats, known as a "lahar", and it can be far more lethal than lava flows. In Colombia in 1985, 24,000 people were buried alive when a lahar swallowed up the town of Armero, following an eruption by the Nevado del Ruiz volcano. Another lahar in 1953 caused New Zealand's worst rail disaster, killing 151 people.
Lahars are caused by a lethal combination of water and volcanic flows. Either an eruption suddenly melts huge quantities of snow and ice, sending it rushing down the mountain's flanks, or a natural dam is formed, trapping water at high altitude until the wall holding it back is ruptured. This is what has happened on Mount Ruapehu, where a huge crater lake, nearly 9,200ft up, is about to burst its banks.