"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."