When most people think of volcanoes, they look to those on land, and that’s perfectly understandable. It’s worth pointing out, though, that around 80 percent of all of Earth’s volcanoes are actually underwater.

Although mostly calm forges of fire, a new Science Advances study highlights that they can sometimes erupt in spectacular fashion and blow everyone away, volcanologists included.

Back in 2012, a submarine seamount – an underwater volcano – named Havre erupted north of New Zealand, on the Kermadec arc. Enormous rafts of pumice were produced that floated atop the waves, and the eruption was so voluminous that scientists at the time began to think that they had never seen anything like it.

They were right. This new study, led by the University of Tasmania, but featuring an international anthem of volcanologists, reminds us that this was the “largest deep-ocean eruption in history”. Not only that, but thanks to a series of expeditions to the submerged site, researchers are now beginning to understand just how strange it was too.

Back in 2012, a gigantic swath of pumice was spotted from above. Pumice is a fairly common volcanic product; it forms when lava cools incredibly quickly and traps plenty of gas bubbles within it. It’s so porous that this solidified fire froth is less dense than water, which is why it floats.

Underwater eruptions often produce plenty of pumice, because the seawater acts as an effective coolant, and there’s plenty of gas to trap. In that sense, the Havre eruption wasn’t unusual – but the fact that it produced a raft 400 square kilometers (about 154 square miles) in area certainly is. That, by the way, is nearly seven times the area of Manhattan.

It was produced at a breakneck speed too, with its total volume appearing in 21.5 hours. This means that, on average, 9,000 tonnes (9,921 tons) of pumice were produced every single second.

“The eruption rate of the raft was equivalent to large, on-land explosive eruptions, which is astounding," study co-author Samuel Mitchell, a PhD student of volcanology at the University of Hawai’I at Mãnoa, told IFLScience.

Rather quickly, volcanologists realized that this eruption was enormous; indeed, it’s been compared to the largest eruption on land in the 20^th Century, the Novarupta (“new eruption”) event in Alaska in 1912 that produced enough magma to fill up 5,400 Great Pyramids of Giza.

Lead author Dr Rebecca Carey, a volcanologist and ARC Research Fellow at the University of Tasmania, told IFLScience that although Novarupta produced far more volcanic material, it did so from a single vent.

"What’s really cool is that the Havre eruption erupted lava from 14 different vents aligned on caldera ring structures that together represent a massive 6-8 kilometer (3.7-5 mile) long rupture of the caldera wall."

In other words, Havre was a bit of a monster in its own right.

The raft wasn’t the entirety of the eruptive contents of the 2012 event, though. The new study estimates that about 75 percent of the total eruptive volume formed the pumice raft, whereas the rest formed a variety of volcanological marvels that remained submerged.

Elongated lava flows known as “tongues” were recently spotted by the submersibles, as were less buoyant, highly fragmented pumice segments. These waterlogged, individual pumice clasts were enormous, with some clocking in at 9 meters (29.5 feet) in length. Plenty of the once-fresh volcanic debris was found mingling around the collapsed remnants of part of the seascape – underwater avalanches triggered by the violence.

The weirdest aspect of the eruption, however, was that it might not have been particularly explosive.

Eruptions on land that produce this much material this quickly are normally linked to explosive eruption styles. Like the most explosive eruptions, Havre also involved rhyolite, a magma that’s millions of times more viscous than water, and one that’s great at trapping gas, which ultimately creates enormous pressure differences between the magma reservoir and the external environment.

Why certain underwater eruptions are explosive and why others aren’t is not well understood. With respect to the giant submerged pumice deposits, the team suspect that the overlying pressure of the water stopped the trapped gas bubbles expanding too rapidly, which suppressed the explosivity of the eruption.

However, the team note that “the eruption style that generated the pumice raft has not been resolved,” also adding that the source site of the raft cannot be identified. Although resembling explosive remnants in some ways, the raft is so unique that it currently has no suitable explanation.

Carey suspects it was actually "effusive – but we are yet to show that conclusively."

For now, it'll remain a fabulous mystery waiting to be solved.

As outstanding as this eruption was, there's a good chance it's not actually unique.

“Whilst this eruption seems like a very rare occurrence, the fact is we have mapped so little of our seafloor in detail,” Mitchell added.

“Our oceans are so vast to monitor, that large rhyolite eruptions on the seafloor may be more common on submarine volcanic arcs than previously thought.”