The Tangiwai disaster was a deadly railway accident that occurred at 10:21 p.m. on 24 December 1953, when a railway bridge over the Whangaehu River collapsed beneath an express passenger train at Tangiwai, North Island, New Zealand. The locomotive and the first six carriages derailed into the river, killing 151 people. The subsequent board of inquiry found that the accident was caused by the failure of the tephra dam holding back nearby Mount Ruapehu's crater lake, creating a rapid mudflow (lahar) in the Whangaehu River which destroyed one of the bridge piers at Tangiwai only minutes before the train reached the bridge. The volcano at Mount Ruapehu was not erupting at the time. The disaster remains New Zealand's worst rail accident.