Over the past few months, a colossal sheet of floating rock has been drifting across the Southwest Pacific headed towards Australia. The strange occurrence has gathered a lot of media and scientific attention since it was first spotted this summer, however, it remained unclear where the pumice raft actually came from. 

Now, a team of scientists from GEOMAR Helmholtz Centre for Ocean Research Kiel has pinpointed its origin: an obscure, nameless underwater volcano just 50 kilometers (31 miles) northwest of the island of Vava'u in Tonga. 

“In the international scientific literature, it appears so far only under the number 243091 or as Volcano F", Dr Philipp Brandl of GEOMAR, first author of the study, said in a statement.

The pumice sheet, made up of thousands and thousands of small pumice stones, first came to light in early August 2019. Thanks to its porous structure, the pumice is able to float, forming a giant raft that stretched for up to 167 square kilometers (64 square miles) across the sea, around double the size of Manhattan.

"On August 9, 2019 we sailed through a pumice field for 6 to 8 hours, much of the time there was no visible water," Shannon Lenz, a sailor who filmed the pumice raft, explains in a YouTube description of their video uploaded at the time. "It was like plowing through a field. We figured the pumice was at least 6inches thick."

Reporting in the Journal of Volcanology and Geothermal Research, researchers have managed to locate the source of the pumice using freely accessible images taken by the ESA satellite Copernicus Sentinel-2. They discovered clear traces of an active underwater eruption on the surface in the Southwest Pacific that paired up perfectly with the resulting pumice overflow.

"The eruption traces fit exactly to Volcano F," states Dr Brandl. “There were only two stations that recorded seismic signals of a volcanic eruption. However, their data is consistent with Volcano F as the origin." 

Pumice is sometimes formed when liquid lava erupts from a volcano into the sea. Gases dissolve in the magma and form bubbles that get trapped as the rock rapidly cools by the chilly seawater.

The pumice raft is still cruising and, judging by its current speed and direction, is expected to hit the Great Barrier Reef region at the end of January or beginning of February 2020. Once it reaches this final destination, the pumice is expected to dump a load of marine microorganisms onto the reef that scientists believe could help to replenish it. 

Remarkably, the largest deep-ocean eruption ever recorded is believed to have occurred in 2012 just north of New Zealand on the Kermadec arc. This too continued huge amounts of pumice – on average, 9,900 tons of pumice was produced every single second – including vast boulders of SUV-sized rocks.