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.

The volcano, used as a setting in the Lord of the Rings films, last erupted 11 years ago, leaving a wall of tephra - a mixture of ash, sandy particles and heavier material - on top of the crater rim. The barrier is already springing leaks as the lake rises, and unless an eruption or earth tremor triggers the lahar first, scientists expect complete collapse within a few weeks from mid-March. The later the rupture, the bigger the lahar will be: the lake is already deeper than the levels that caused floods in 1953 and 1975, and this time more than a million cubic metres of mud and rocks will career down the mountain. TV companies are fighting for exclusive rights to film the spectacular event.

The rare ability to predict a natural disaster means that the authorities insist there should be no danger this time. The steady rise of the lake has been closely monitored over the past decade, and millions of dollars have been spent on safety measures. A sophisticated early-warning system will be triggered by any event, causing gates to close bridges and roads in the flood's path. A specially assigned team of "lahar cops" is on 24-hour call.

While not in any immediate danger, guests at the exclusive Chateau Tongariro resort will be evacuated if the alert goes up. "You have to realise we live and work on the flanks of a volcano, so this is second nature to us," said the general manager, Kathy Guy.

The Christmas Eve lahar in 1953 washed out a rail bridge over the river as an express train was approaching. Although a local man rushed up the track with a torch to warn the driver, he could not stop in time, and the first six coaches fell into the flood.

The main concern of the authorities this time is the threat to tourism, New Zealand's main industry. The last major eruption of the world heritage-listed volcano, in 1995, caused considerable harm to the economy. "Tourist groups were cancelling, as they thought it wasn't safe," said Dr Harry Keys, a Department of Conservation scientist. "There were reports the North Island was being evacuated to Australia. One story, out of Europe, had the whole country reduced to a few steaming islands."

http://news.independent.co.uk/world/australasia/article2326235.ece