The analysis of teleseismic travel time residuals recorded on the Larderello Seismic Network of central Italy has revealed a sharp low velocity zone (LVZ) in the center of the geothermal production area. Residuals from 101 teleseismic events are inverted to produce an image of a region of anomalously low velocities which we have interpreted to be a series of intrusive magmatic bodies. The top of this anomalous region is constrained to be below 6 km in depth from local earthquake seismicity patterns and extends to a depth of greater than 20 km. The LVZ broadens with increasing depth and may extend into the upper mantle. The observed travel time residual anomaly correlates spatially with observed gravity lows, regions of high heat flow, and the shallowing of a dominant upper crustal reflection horizon. This body has elevated the temperatures in the region to over 350° C at 2 km and appears to be the origin of the geothermal energy