Among geologists who study powerful earthquakes and volcanoes, there is a mystery: as one of Earth’s tectonic plates slides beneath another in a subduction zone, water is squeezed from certain minerals, lubricating earthquakes and fueling volcanoes in hot spots like the Pacific Ocean “Ring of Fire.” But equations that predict where the forces of subduction wring water from stone consistently point to locations far from the site of actual cataclysms.