The ring plain around the central volcanoes is formed from numerous lahars or mudflows that occur when either the crater lake water is ejected from Ruapehu or snow is melted, during an eruption. The water runs down the mountain, picking up ash, lapilli, blocks and bombs as well as previously eroded rock, forming a thick, dense mud that is capable of picking up car-sized rocks. An old lahar flow is visible from the approach road to Whakapapa ski field. In December 1953 a sudden lahar at night on Mt Ruapehu knocked out the rail bridge at Tangiwai, causing six carriages of the Wellington-Auckland express to plunge into the Whangaehu River, killing 151 people.