One of the unsolved problems in current physics  is a mechanism of sonoluminescence. Sonoluminescence is a phenomenon that occurs when a small gas bubble is acoustically suspended and periodically driven in a liquid solution at ultrasonic frequencies, resulting in bubble collapse, cavitation, and light emission . For example in a single-bubble sonoluminescence (SBSL), a single bubble is trapped in an acoustic standing wave and emits bursts of light with each compression of the bubble. It was realized that temperatures inside bubbles can reach unbelievable values of thousands of Kelvins.
The mechanism of sonoluminescence however remains unsettled. There are plenty of theories including hotspot, bremsstrahlung radiation, collision-induced radiation, corona discharges, nonclassical light, proton tunneling, etc. . Exotic mechanism proposed by Julian Schwinger (Nobel prize for Quantum Electrodynamics, 1965) and followed by the others suggests a quantum vacuum radiation as an explanation.