Water dragged into Earth’s interior helps melt rock, but near the Tonga trench there’s the least magma where there’s the most water. A three-dimensional seismic image of the mantle beneath the Lau Basin in the South Pacific has an intriguing anomaly. The image showed the least magma where the scientists expected to find the most. After considerable debate they concluded that magma with a high water content was flushed so rapidly that it wasn't showing up in the images....