Sonoluminescence is the emission of short bursts of light from imploding bubbles in a liquid when excited by sound. Discovered in 1934 during sonar experiments at the University of Cologne. Light emissions are the result of the ultra-high temperatures caused as collapsing bubbles generate an imploding shock wave that compresses and heats the gas at the centre of the bubble.
Measurements suggest extremely high temperatures - estimated to be from 2,300 K to 20,000 K.
The mechanism of sonoluminescence is unknown - though there are several as-yet-unconfirmed theories.