We All Nearly Missed the Largest Underwater Volcano Eruption Ever Recorded

schwit1 quotes ScienceAlert:
She was flying home from a holiday in Samoa when she saw it through the airplane window: a "peculiar large mass" floating on the ocean, hundreds of kilometres off the north coast of New Zealand. The Kiwi passenger emailed photos of the strange ocean slick to scientists, who realised what it was -- a raft of floating rock spewed from an underwater volcano, produced in the largest eruption of its kind ever recorded.

"We knew it was a large-scale eruption, approximately equivalent to the biggest eruption we've seen on land in the 20th Century," says volcanologist Rebecca Carey from the University of Tasmania, who's co-led the first close-up investigation of the historic 2012 eruption. The incident, produced by a submarine volcano called the Havre Seamount, initially went unnoticed by scientists, but the floating rock platform it generated was harder to miss. Back in 2012, the raft -- composed of pumice rock -- covered some 400 square kilometres (154 square miles) of the south-west Pacific Ocean, but months later satellites recorded it dispersing over an area twice the size of New Zealand itself... for a sense of scale, think roughly 1.5 times larger than the 1980 eruption of Mount St. Helens -- or 10 times the size of the 2010 Eyjafjallajokull eruption in Iceland.
When an underwater robot first sent back detailed maps, one volcanologist remembers that "I thought the vehicle's sonar was acting up... We saw all these bumps on the seafloor... It turned out that each bump was a giant block of pumice, some of them the size of a van."