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My favorite example of these water experiments is sonoluminescence: a tank of water is driven by a high-frequency acoustic transducer to create a 3d standing wave, which forms a tiny bubble of air or other gas that, when collapsing, produces a burst of light.

That tiny burst of light is still an unsolved mystery. Despite it's trivially reproducible, there's still a range of competing theories that estimate the temperature of that bubble from 2,000K to 20,000K (some researchers add a few zeros there), and attribute the light to anything imaginable, including fusion and the Casimir effect. Quoting Wikipedia: "the rapidly moving interface between water and gas converts virtual photons into real photons."