Ruapehu Crater Lake finally bursts.

[Jethro Gibbs]

A potentially lethal mix of mud, acidic water and rocks tore down the slope of New Zealand's Mount Ruapehu on Sunday, emergency officials said.

There was no immediate threat to life and no reported injuries.

The mud flow, also known as a lahar, broke through a rubble wall atop the mountain's crater lake about 10:47 a.m. local time Sunday, triggering an early-warning alarm, Horizon regional council said.

"It didn't take long until all the boulders just disappeared and you could hear the rumbling as they moved with the volume of water," an unidentified local resident said.

On Dec. 24, 1953, a lahar tore down the same volcano, killing 151 people when it washed away a rail bridge and a passenger train plunged into the raging torrent of liquid mud. It became known as the Tangiwai disaster.

Police and civil defence workers immediately closed roads and the nation's main trunk rail track near the southern base of the mountain on New Zealand's North Island