In the beginning of the 20th century, Louis de Broglie suggested that, analogous to light, matter too had both particle and wave properties. Several experiments where crystals were bombarded by electrons showed diffraction patterns, supporting this theory. However, one of the most interesting experiments was done by C. Jönsson in 1961.
Jönsson repeated Young's double slit experiment, but instead of using light he used a beam of electrons. The same interference pattern was observed, and the wavelength of the electrons was found to agree with the de Broglie wavelength.
One would think that a beam of mutually interferencing electrons would give rise to the observed pattern, but that a single electron would not be able to do so.
Later, the Jönsson experiment was repeated with an electron source that emitted only a few electrons per hour.
Using photographic plates and extremely long exposure times, the interference effect was still found to occur.
Even today this effect boggles the mind of physicists.