In 1982 and 1984 the US Geological Survey used several seismic networks, totaling over 90 stations, to record teleseismic P waves and measure travel time residuals in an area centered on the Long Valley caldera. The travel time residuals have been inverted to obtain a three-dimensional image of the velocity structure with resolution of 5-6 km to depths of 70 km beneath the array. Direct inversion of these data indicates that the 2- to 4-km-thick low-velocity caldera fill contaminates the signal from any midcrustal velocity anomalies beneath the caldera. Two methods were used to strip the effects of the upper crust from the travel time residuals and the resulting "stripped' models show two well-resolved midcrustal low-velocity bodies in the Long Valley region. The features are interpreted as silicic magma chambers and the presence of additional pockets of magma <5 km across in the upper crust is not ruled out. The high eruptive rate of the Mono Craters and upper mantle velocity anomalies suggest that the focus of volcanism is shifting north from Long Valley to the Mono Craters. -from Authors