Global seismic tomography has revealed the existence of a now-stagnant subducted slab (Figure 1c, inset) in the mantle transition zone (MTZ) of the western Pacific Ocean [e.g., Fukao et al., 2001], where the old Pacific plate subducts at the Kuril/Japan/Mariana Trench system. Fukao et al.  showed that a stagnant slab is a common feature of many subduction systems around the world, although it is still not well understood why and how a subducted slab may or may not become stagnant. In addition to a subducted slab itself, water entrained with the slab may also accumulate in the MTZ in association with stagnation. The MTZ has been thought to play a key role in the Earth's water cycle, with water transported to the MTZ possibly being released and supplied to the upper mantle to drive back-arc volcanism [e.g., Ichiki et al., 2006].