What they observed is explained by Gerald Pollack's work. He discusses "cavitation bubbles" in his book The Fourth Phase of Water. He argues, from evidence, that the bubble consists of an EZ membrane that surrounds a core containing protons or hydronium ions. The protons push against the EZ membrane, creating sphericality. Since the EZ contains negative charge, while the core contains positive charge, the potential energy is huge. Upon bubble collapse, that energy is released. The 4th phase powers the phase change into that 5th (or 6th if plasma is included) phase of water.