The curious, and thus far unexplained, phenomenon of sonoluminescence was discovered in the 1930s, but extensive experiments have only been conducted in the past few decades, with the Putterman Group at UCLA featuring prominently. When a bubble sits in a fluid medium, e.g. water, an acoustic standing wave traps the bubble at an anti-node, keeping the bubble fixed in position in the water, as you can see below (the little blue dot occurs at the position of the bubble):
What then happens is rather spectacular. The bubble, sitting at the anti-node, endures oscillations in its radius due to the compression and rarefaction of the surrounding water. When the bubble collapses (about 40,000 times per second), it emits light each time! The light can even be seen with the naked eye in the laboratory. Here is a rather stunning movie of the phenomenon:
It was subsequently deduced by Flannigan and Suslick (pdf!) that the temperature inside the collapsing bubble is approximately a whopping 20,000 Kelvin (as a reference, the surface of the sun is ~6000 Kelvin). This has led to speculations about atomic fusion inside the bubble, but no evidence for extra neutron production exists. As with any claim of tabletop fusion, there exist allegations of data falsification related to this.
I was lucky enough to study this phenomenon in an undergraduate lab, and it must be said that experiments like these had an impact in making me decide to become an experimental physicist.