In this film clip, New Zealand geologist Charles Cotton describes the effects of the 1945 eruption of Mt Ruapehu. At the time it seemed there would be little danger to the public because Ruapehu's crater was so isolated.

Once volcanic activity subsided the crater lake gradually refilled over the next eight years. On 24 December 1953 a lahar (volcanic mud flow) swept away the rail bridge at Tangiwai, leading to the deaths of 151 people. Cotton did not foresee such an event – but neither did anyone else at the time.