The ultra-low frequency band (2-8 Hz) is of interest for geophysical research due to advances in the field of full waveform inversion and elastic impedance measurements. Generating sound in the ultra-low frequency range is a difficult task. A powerful source with an ultra-low frequency should be able to displace hundreds of liters of water per cycle. The amplitude of internal pressure fluctuations is comparable to the difference in buoyancy forces on the radiation aperture, and acoustic-gravitational effects are part of its hydrodynamics. The source described in this article has a pneumatically driven bubble resonator and provides a volume displacement and radiation area that are larger than other known prototypes. The article examines the acoustic physics of a large underwater bubble resonator and a seismic bubble source with an internal Helmholtz resonator. A finite element analysis of the transition of near-field hydrodynamics to a pressure wave is included, as well as a treatment of transition loss and broadband radiation methods. The study concludes with the creation and testing of an ultra-low frequency seismic source. Experiments carried out at the Woods Hole Oceanographic Institution showed that the prototype has a source level high enough for full waveform inversion in geophysical surveys.