And if you want to know what each of those little bubbles is doing when it collapses then try this image. You can see from this high-speed camera that the middle of the bubble jets down through the middle of the bubble. It is as this jet hits the solid surface underneath that the characteristic pit marks are made as this jet can be traveling at incredible speeds. This frame is taken just before the bubble collapses completely.
Incredibly the collapse can be so violent with enough heat generated to cause a pulse of light to be emitted - this is sonoluminescence.
Now we know why those bubbles can be such destructive little b*ggers
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