Sonoluminescence is a still not completely understood physical/chemical phenomen which causes the emission of short bursts of light from imploding bubbles in a liquid when excited by sound. This effect was first discoverd 1934 as a result of work on sonar. Two scientists put an ultrasound transducer in a tank of photographic developer fluid and they noticed tiny dots on the film. They realized that the bubbles in the fluid were emitting light with the ultrasound turned on. This phenomenon is now referred to as multi-bubble sonoluminescence. Until then many other experiments followed and different scientists were struggeling to find an explantion for the light emission. It was found out that the temperature inside the bubbles can be hot enough to melt steel but other scientists even claimed that under certain circumstances the inner temperature of such a bubble can reach up to million kelvins. However, as the temperature inside the bubble can not be measured directly nobody has confirmed such high temperatures until now and recent experiments indicate temperatures around 20,000 K. The basic physics of the process seems to be understood but a consistent description of the whole mechanism of sonoluminescence still remains unsettled. Thus, I am not going to summarize all theories that deal with this pheonmen, but just refer to this Nature article where you can find more details and references. Although, we are living in the 21st century there are still many unsolved physical problems and who would have though that water bubbles are one of them?