A lahar ( , from Javanese: ) is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley.Lahars are extremely destructive: they can flow tens of metres per second, they have been known to be up to 140 metres (460 ft) deep, and large flows tend to destroy any structures in their path. Notable lahars include those at Mount Pinatubo and Nevado del Ruiz, the latter of which killed thousands of people in the town of Armero.

The Tangiwai disaster 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 collapse of the tephra dam holding back nearby Mount Ruapehu's crater lake, creating a large lahar in the Whangaehu River, which destroyed one of the bridge piers at Tangiwai only minutes before the train reached the bridge. The volcano itself was not otherwise erupting at the time. The disaster remains New Zealand's worst rail accident.