Hollow reentrants in quartz phenocrysts from Yellowstone (western United States) caldera’s Lava Creek Tuff are preserved vestiges of bubbles in the supereruption’s pre-eruptive magma reservoir. We characterized the reentrants using a combination of petrographic techniques, synchrotron X-ray microtomography, and cathodoluminescence imagery. One or more reentrants occur in ∼20% of quartz, and up to ∼90% of those reentrants are hollow. The earliest-erupted parts of the Lava Creek Tuff have the most empty reentrants. The hollow reentrants provide direct, physical evidence for volatile saturation, exsolution, and retention in a magma reservoir. Quartz-melt surface tension permits bubbles to attach to quartz only when bubbles have been able to nucleate and grow in the melt. Prior to eruption, the Lava Creek Tuff existed as a bubbly, volatile-saturated magma reservoir. The exsolved volatiles increased magma compressibility, helping to prevent the ever-accumulating magma from reaching a critical, eruptive overpressure until it reached a tremendous volume.