These lava lakes drained catastrophically—and scientists caught it in action

When Yves Moussallam trekked around Vanuatu’s Ambrym volcano in the winter of 2018, the ground was blanketed in green, and five incandescent lakes of molten rock burbled in the volcano’s caldera. Just two weeks later, though, he found himself in a landscape devoid of color. Gray ash coated each rock and crevice, and the lakes sat empty, their lava vanished like water swirled down a drain.

“It looked like everything was in black-and-white,” says Moussallam, a volcanologist at Columbia University who is also associated with France’s Laboratoire Magmas et Volcans. “The whole caldera area had completely changed.”

This transformation came in the wake of an extraordinary eruption that surprised scientists with its progression. While some of the lava spurted up from nearby cracks, the vast majority moved underground—a slug of magma big enough to fill 160,000 Olympic swimming pools. As the team reports in Scientific Reports, the process cracked the earth, sending coasts soaring into the air, and brought lava burbling up onto the ocean floor.

“It’s kind of a negative eruption, in a way,” says volcanologist Clive Oppenheimer of the University of Cambridge, who was not on the study team. “It’s not stuff coming out of the ground, it’s the magma migrating beneath the ground.”

The new study, conducted in collaboration with the Vanuatu Meteorology and Geohazards Department, provides a rare and detailed portrait of Ambrym’s activity above and below, which can help geologists unravel the myriad processes that contribute to volcanic activity.

“As volcanologists, we’re always trying to...