A high intensity ultrasonic wave causes acoustic cavitation.  As the rarefaction phase of the wave passes through the bubble, the inside experiences a near vacuum.  A cavitating bubble has several occurrences of expansion and contraction until there is one final extreme implosion where the temperature in the center is hotter that the surface of the sun.  Often there is a flash of light, and if there are many cavitating bubbles, there can be many flashes of light.

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