Mount Ruapehu in New Zealand is the most active volcano in the world. Major eruptions occur about every fifty years, with 60 or more minor eruptions since 1945. To add to this hazardous situation, the volcano’s crater periodically fills with water between eruptions causing the crater wall to collapse and release flood water and mud down the mountain onto inhabited areas. On Christmas Eve 1953 such a flood, called a lahar, washed an overnight train off a bridge, leading to the loss of 153 lives.