Magnetotelluric data have been increasingly used to image subduction zones. Models of electrical resistivity commonly show features related to the release of fluids at several depths through the systems imaged, consistent with thermal and petrologic models of dehydration of the downgoing slab. Imaging the release of fluids from sediments and pore space in the crust requires controlled source electromagnetic techniques, which have to date only been used in one setting, offshore Nicaragua. The release of fluids related to the transition of basalt to eclogite is commonly imaged with magnetotelluric data. Deeper fluid release signals, from the breakdown of minerals like serpentine, are highly variable. We hypothesize that regions where very strong conductive anomalies are observed in the mantle wedge at depths of ∼80–100 km are related to the subduction of anomalous seafloor, either related to excessive fracturing of the crust (e.g., fracture zones), subduction of seamounts, or other ridges and areas of high relief. These features deform the seafloor prior to entering the trench, permitting more widespread serpentinization of the mantle than would otherwise occur. An alternative explanation is that the large conductors represent melts with higher contents of crustal-derived volatiles (such as C and H), suggesting in particular locally higher fluxes of carbon into the mantle wedge, perhaps also associated with subduction of anomalous seafloor structures with greater degrees of hydrothermal alteration.