Restless Ruapehu a bigger threat
Just like Mt Tongariro's surprise bang on Wednesday afternoon, what happened a few kilometres away at 8.20pm on September 25, 2007, came suddenly and violently.
Shortly before airline pilots noticed a black plume rising above Mt Ruapehu, a volcanic blast threw ash, rocks and water across the summit area, sending two muddy torrents down the skifields.
Inside a hut on the edge of the crater lake, William Pike and James Christie heard a "massive boom" before the building's door was blown from its hinges and mud and rock poured inside. Mr Pike's crushed leg later had to be amputated.
The warning signs Mt Ruapehu gave in the days before that explosive moment are being seen again now - worrying scientists that the mountain could be about to produce a similar-sized eruption.
GNS volcanologist Michael Rosenberg said Mt Ruapehu has been showing two forms of unrest, which are considered unrelated.
There have been 45 earthquakes about 5km beneath the mountain since early August, but 35 of those have come in the past month......
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