Last month, the_ HMS Canterbury_, a support ship in the New Zealand navy, stumbled across what one of its officers called “the weirdest thing I’ve seen in 18 years at sea.” While sailing southwest of Raoul Island, the largest of the Kermadec archipelago in the South Pacific between New Zealand and Tonga, the crew of the Canterbury saw something completely unexpected: a new island nearly 300 miles long, not located on any chart but suddenly lying dead ahead.

View the area where the Havre Pumice Raft was spotted in a larger map

The Canterbury saw that the enormous new island was also “moving up and down on the waves,” giving them the best clue what it was suddenly doing there. This was a pumice raft, a floating piece of porous volcanic rock. Pumice rafts are a not-uncommon result of undersea volcanic eruption far out at sea: Lava cools when it hits the cold ocean water, and suddenly a mass of rock the size of New Jersey just bobs out of the middle of the ocean.

Later that week, NASA scientists poring through satellite images determined the new island’s birthplace: the Havre Seamount, an underwater volcano near the southern tip of the Kermadecs. Havre is so obscure that many volcanic maps and databases don’t even include it, but NASA satellites had caught the telltale plume of ash boiling from the sea three weeks earlier.

Pumice rafts are actually tightly packed collections of trillions of individual pumice rocks. The newborn one spotted by the Canterbury was reported to stand two feet above the water, meaning it was probably solid enough for people to walk on. But not for long: more recent satellite images of the Havre pumice raft show it dispersing across the South Pacific leaving vast streams of rock and foam in its wake. In its first month alone, the new “island” spread out over an area of over 100,000 square miles.

A similar raft appeared in the South Pacific after a 2006 eruption.

Since the island is now dispersing too fast to appear on any map, I’m not sure if it qualifies for one of these “Maphead” posts or not. But even if it’s too late for you to visit the Havre pumice raft, it might visit you one of these days. A similar raft appeared in the South Pacific after a 2006 eruption, and its various pieces were still afloat and washing up on beaches thousands of miles away two years later! Often these pieces of pumice were found encrusted with barnacles, corals, seaweed, and other ocean organisms. Scientists think that, over the geologic ages, lots of plants and animals probably colonized new oceans and shores by hopping a ride on pumice rafts.

From top: NASA image courtesy Jeff Schmaltz LANCE/EOSDIS MODIS Rapid Response Team, GSFC; Visuals Unlimited/Corbis