A little more than 760 ka ago, a supervolcano on the eastern edge of California (United States) underwent one of North America’s largest Quaternary explosive eruptions. Over this ∼6-day-long eruption, pyroclastic flows blanketed the surrounding ∼50 km with more than 1400 km3 of the now-iconic Bishop Tuff, with ashfall reaching as far east as Nebraska. Collapse of the volcano’s magma reservoir created the restless Long Valley Caldera. Although no rhyolitic eruptions have occurred in 100 k.y., beginning in 1978, ongoing uplift suggests new magma may have intruded into the reservoir. Alternatively, the reservoir could be approaching final crystallization, with present-day uplift related to the expulsion of fluid from the last vestiges of melt. Despite 40 years of diverse investigations, the presence of large volumes of melt in Long Valley’s magma reservoir remain unresolved. Here we show, through full waveform seismic tomography, a mid-crustal zone of low shear-wave velocity. We estimate the reservoir contains considerable quantities of melt, >1000 km3, at melt fractions as high as ∼27%. While supervolcanoes like Long Valley are rare, understanding the volume and concentration of melt in their magma reservoirs is critical for determining their potential hazard.