In conjunction with the USArray component of EarthScope, long period magnetotelluric (MT) data are being acquired in a series of arrays across the continental US. Initial deployments in 2006 and 2007 acquired data (10–10,000 s) at 110 sites covering the US Pacific Northwest, distributed with the same nominal spacing as the USArray seismic transportable array (∼75 km). The most striking and robust features revealed by initial three-dimensional inversion of this dataset are extensive areas of high conductivity in the lower crust beneath all of southeastern Oregon, and beneath the Cascade Mountains, contrasting with very resistive crust in Siletzia and the Columbia Embayment. Significant variations in upper mantle conductivity are also revealed by the inversions, with the most conductive mantle beneath the Washington backarc, and the most resistive corresponding to subducting oceanic mantle.