The ocean crust is the largest aquifer on earth, and it hosts vigorous fluid circulation – the entire volume of the world’s oceans cycles through the seafloor in less than 500,000 years. This fluid circulation plays an important role in redistributing heat within the ocean crust, and can control the crust’s thermal state by extracting heat.  The thermal state of the crust, in turn, affects metamorphic and geodynamic processes.   For example, the thermal state of ocean crust is an important control on subduction zone temperatures, which may affect the updip and downdip limits of subduction zone earthquakes.  New subduction zone thermal models demonstrate that hydrothermal circulation in subducting ocean crust can greatly suppress temperatures along the subducting slab.  Hydrothermal circulation reduces temperatures in the Nanaki margin (southern Japan) subduction zone plate boundary fault by ~20 °C at the updip limit of seismicity and ~100 °C at the downdip limit. Metamorphic reactions in the subducting slab are delayed (i.e. shifted landward) relative to a case with no fluid circulation.