That's how you prevent a major disaster. Brilliant planning.
http://www.boston.com/news/world/australia/articles/2007/03/18/mud_rocks_rush_from_new_zealand_volcano/<snip>
A potentially lethal mix of mud, acidic water and rocks tore down the slope of New Zealand's Mount Ruapehu on Sunday, emergency officials said, but there was no immediate threat to life.
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Police and civil defense workers immediately closed roads and the nation's main trunk rail track near the southern base of the mountain on New Zealand's North Island.
The island's main north-south highway, some 30 miles from the mountain's base, also was closed, and two passenger trains with 200 people on board were halted some distance from the mountain.
A lahar that tore down the same volcano in 1953 killed 151 people when it washed away a rail bridge, plunging a passenger train into the raging torrent of mud.
A naturally occurring 23-foot wall of volcanic ash and sand known as tephra sits atop the mountain, holding in millions of gallons of acidic water, and this had been breached, regional council chairman Gary Murfitt said.
"It is on its way down ... and the good thing is that rivers in that area are not affected by flooding, so it will only be the lahar going down," he said.
Scientists had been able to predict the lahar's passage and an early warning response system had worked as planned, Conservation Minister Chris Carter said.
"The safety systems, the bund (earth dam), the strengthening of the (rail) bridge, the early warning systems have all worked perfectly," he said, adding the local "community had been kept safe."