Surface wave tomography of the western United States reveals the presence of an unusually slow shear wave velocity anomaly beneath a region in the Basin and Range Province, sometimes termed the amagmatic zone, which is renowned for its lack of volcanic activity. We present a model of the three-dimensional shear wave velocity structure beneath the southwestern United States constructed from the inversion of fundamental mode Rayleigh wave dispersion. The abundance of data from the deployment of the USArray (a component of the EarthScope project) provides unprecedented resolution. There is an excellent correlation between the location of slow shear wave velocity anomalies and recent (younger than 1 Ma) volcanism. This correlation and the unusually low values of absolute shear velocity beneath the magmatic gap lead us to conclude that there is melt present beneath this region. Previous studies have proposed that the preservation of cold continental lithosphere beneath the magmatic gap accounted for the lack of volcanism; however, this hypothesis is inconsistent with anomalously slow shear wave velocities in the 50–100 km depth range. Perhaps the prevalence of low-angle normal faulting in this region has made it difficult for the melt to escape the mantle. Beneath an immediately adjacent region, the southwestern Nevada volcanic field, where volcanism was abundant 10–15 m.y. ago but has since waned, significantly higher shear velocities suggest that the melt has been extracted from the mantle.