Detection of three very-long-period (VLP) volcanic earthquakes beneath Mammoth Mountain emphasizes that magmatic processes continue to be active beneath this young, eastern California volcano. These VLP earthquakes, which occurred in October 1996 and July and August 2000, appear as bell-shaped pulses with durations of one to two minutes on a nearby borehole dilatometer and on the displacement seismogram from a nearby broadband seismometer. They are accompanied by rapid-fire sequences of high-frequency (HF) earthquakes and several long-period (LP) volcanic earthquakes. The limited VLP data are consistent with a CLVD source at a depth of ~3 km beneath the summit, which we interpret as resulting from a slug of fluid (CO2--saturated magmatic brine or perhaps basaltic magma) moving into a crack