Interpretation of the EarthScope MT (magnetotelluric) data requires the development of a large-scale inversion method which can address two common problems of 3D MT inversion: computational time and memory requirements. We have developed an efficient method of 3D MT inversion based on an IE (integral equation) formulation of the MT forward modeling problem and a receiver footprint approach, implemented as a massively parallel algorithm. This method is applied to the MT data collected in the western United States as a part of the EarthScope project. As a result, we present one of the first 3D geoelectrical images of the upper mantle beneath Yellowstone revealed by this large-scale 3D inversion of the EarthScope MT data. These images show a highly conductive body associated with the tomographically imaged mantle plume-like layer of hot material rising from the upper mantle toward the Yellowstone volcano. The conductive body identified in these images is west-dipping in a similar way to a P-wave low-velocity body.