The red pointer shows where the Crater Lake on Mt Ruapehu gave way forming a lahar which cascaded down the mountain. Photo / Greg Bowker

DoC have confirmed a moderate lahar is underway, after alarms were activated on Mt Ruapehu's crater lake today.

But what is a lahar - is it the same as a volcano? What causes them, and, more importantly, how prepared are we?

Here is a series of previous stories the Herald has done analysing a lahar from Ruapehu.

PHOTOS: Scientists polish up crisis threat for lahar

Lahar alert rises

Crater under surveillance

First testing of the lahar alarm system, from 2004