Subduction zones, where one tectonic plate slides beneath another, produce the most devastating seismic, volcanic, and landslide hazards on the planet. These geohazards pose a significant risk to human population centers, disrupting entire societies. Major advances in understanding the science behind subduction zone hazards could yield tangible benefits for communities in the affected regions, including the possibility of useful forecasts of large earthquakes, volcanic eruptions, and landslides. Yet, we have only limited critical understanding of the complex physical and chemical processes controlling the occurrence, timing, and magnitude of earthquakes, tsunamis, and eruptions.