We all know how sound is composed of vibrations in the air, and that sound recording is possible when these vibrations affect the diaphragm of a microphone. But what if the use of a microphone is a bit awkward, as in the case of spying? Among the scientific geniuses that we seemed to have a lot of years ago, Léon Theremin (inventor of the creepy-sounding musical instrument bearing his name) did not think a microphone was necessary; he developed a system called the Buran eavesdropping system for the Soviets at the end of World War 2. It would use an infra-red powered beam and when pointed at a window, it could detect vibrations in the window which revealed the sounds of the room behind it. Fast forward a few decades, and this technique has been improved upon somewhat. With the computer power available now, silent video recordings can be studied for tiny vibrations (100/th of a pixel), and the sound at the location can be reconstructed. The video below shows that this can be achieved, although it's not strictly hi-fi!