Four ocean bottom electromagnetometers provided seafloor geoelectromagnetic data for more than nine months in order to reveal mantle dynamics beneath the eastern margin of Japan Sea, where several large and hazardous earthquakes have repeatedly occurred so far. High-quality magnetotelluric (MT) data from the seafloor array, together with those of four additional land sites in northeast Japan, were analyzed and carefully corrected for topographic/bathymetric effects to extract two-dimensional (2D) MT responses at each site. Here we show how the 2D electrical section derived by inverting the corrected 2D MT responses depicts fluid injection deep into the wedge and back-arc mantle by the cold and fast subducting Pacific plate. A high conductivity anomaly found beneath the Sado Ridge at depths ranging from 150 to 200 km may be direct field evidence that supports water transportation from the Earth's surface to the wedge and back-arc mantle in the form of serpentinite.