If you collapse an underwater bubble with a soundwave, light is produced, and nobody knows why.
It’s a phenomenon called sonoluminescence. Sonoluminescence is a physical occurrence by which sound turns into light. Scientists have been trying for 70 years to explain it, but have had no success. No one has managed to explain how a bubble of air in water can focus sound to cause light, but it happens.
Some minor revelations have surfaced, however. At first, physicists thought friction was to blame, but in the late 1980s, they discovered that a sound wave’s path expanded bubbles and heated the gas inside them to temperatures hotter than the sun’s surface. That collapse with the heat, they thought, created a glowing plasma. Thirty years later, that is still the going theory.
However, researchers have suggested that different physical mechanisms must be at work and that there must be multiple kinds of sonoluminescence. What’s been concretely determined so far, though, is that it has to do with the size of the bubble as well as the OH emission from the bubble when it bursts.
If the science goes much further, it could be possible that some day sound and gas could be used to light underwater areas, exceeding the limitations of conventional lights.