We analyze swarms of exceptionally intense and sustained T waves recorded during 1991 through 1994 at the Polynesian seismic array. The strongest swarm lasted 15 months and originated southwest of the Eltanin fracture zone, in the immediate vicinity of a documented seamount associated with a recognized volcanic chain branching out of the local segment of the mid-oceanic ridge. The Eltanin T waves are characterized by an exceptionally monochromatic spectrum featuring a single line (of a few Hertz frequency) in the range 2 to 80 Hz. While no similar characteristics had previously been observed in 25 yr of recording in Polynesia, comparable spectra were recorded during a 1993 swarm at the Revilla Gigedo Islands, following by 4 months a documented eruption of Socorro Island. In both cases, we interpret the T waves as evidence of a major underwater volcanic process. The existence of a single resonating frequency, and in particular the absence of any overtones, is generally not compatible with the now classical models of the resonance of a fluid-filled crack, and we speculate that the source of the phenomenon may be the oscillation of a bubbly liquid, which could result from vaporization of seawater in the presence of a large lava lake.