Earth’s largest volcanic system, hidden in mountain chains under the sea, has long been assumed to erupt only quietly. The shallow seafloor off Iceland tells another story. Rock formations emerge from the waves along the shore of Iceland. Jonas Preine Introduction Jonas Preine, a recently minted Ph.D. from the University of Hamburg, squinted at a computer screen in the lab of a ship as it bobbed in the North Atlantic near Iceland. The image before him just didn’t make sense. It was June 2024, and Preine was among a crew of scientists who had set off from Reykjavik under slate-colored skies, trading their regular lives — family, friends, and the typical office environment — for cramped quarters and nausea on board the Meteor, a research vessel chartered for Expedition M201. They’d been lucky so far, enjoying relatively calm seas as they motored toward their destination, an unexplored deep-water basin dotted with volcanic shapes.…