Sonoluminescence is the emission of very short (pico-second) pulses of light from imploding bubbles in a liquid when it's subjected to strong sound waves. It was discovered in 1934 during sonar experiments at the University of Cologne.

It was quickly determined that the light emissions were the result of the ultra-high temperatures caused as collapsing bubbles generate an imploding shock wave which compresses and heats the gas into a plasma (at the centre of the bubble).

Light spectrum measurements point to extremely high temperatures - estimated to be from 2,300 K to 20,000 K.

The exact mechanism of sonoluminescence is still unknown - though there are several as-yet-unconfirmed theories.

The two most prominent theories are currently the Shock Wave model and the Hot-Spot model.

[…] the shock wave model that a spherical shock wave converges at the bubble center where extremely high temperature plasma is formed. The other is the hot-spot model that nearly the whole bubble is heated by quasi-adiabatic compression, where ‘quasi-‘ means appreciable thermal conduction takes place between the heated bubble interior and the surrounding liquid.

Source : Multibubble Sonoluminescence from a Theoretical Perspective Molecules 2021, 26(15), 4624

Technical details of other theories are described at Wikipedia

Recent (2022) research from the University of Ottawa found that the statistical analysis of the photon emissions suggests that they are due to some kind of quantum level effects - though the exact nature of the process remains unclear. See Observation of Nonclassical Photon Statistics in Single-Bubble Sonoluminescence arXiv.org, 2022

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