The theremin may be the most obscure instrument in modern music. It was used by the Beach Boys on some early sessions.
Now someone is selling a homemade theremin on eBay UK made out of a vegemite jar. The instrument is sensitive to light or the magnetic presence of objects. One can wave their arm or any object around the instrument and it emits a malleable sound wave sound reminiscent of early satellites and old science fiction shows.
Even a search for "theremin" on Google doesn't produce much concrete background on this instrument, but there's a whole Theremin World out there somewhere.