Underwater volcanic eruptions result in unique products and hazards, but remain poorly understood. This project will reveal the depth and volume of magma stored beneath 60 underwater volcanoes. A novel method will be used detect magma beneath the entire Kermadec-Tonga arc from gravity anomalies derived from new satellite altimetry data. The unprecedented spatial scale and high resolution of the data will answer longstanding questions. Examples include what controls the depth of the magma chamber and the relationship between eruption frequency and magma storage volume. The project will create open-access scientific software. It supports an early-career PI and training of a postdoctoral scientist. The results will be shared with the public through a digital atlas of submarine volcanoes. Submarine volcanism results in unique eruption products and geohazards that remain poorly understood. This project will use the new Surface Water Ocean Topography satellite altimetry data to calculate gravity anomalies associated with 60 underwater volcanoes along the Kermadec-Tonga arc and derive 3D images of their magmatic systems. The significance of the gravity anomalies in relation to magma volume, hydrothermal fluid, and magma storage under these volcanoes will be investigated by using a novel geophysical inversion method. The new data will address key questions about what controls magma storage depth and volume. Eight active calderas along the arc where geomagnetic, ship gravity and ge