The answer is – theremin.  Invented by Russian Léon Theremin in 1919, it consists of two radio frequency oscillators and two metal loop antennas. The electric signals from the theremin are amplified and sent to a loudspeaker.

 

The theremin is widely associated with "alien" or surreal sounds, and has been used in films like Spellbound, The Lost Weekend, and The Day the Earth Stood Still, to name a few.

 

The following 2 minute recording from a radio interview by Robert Moog, describes how theremin works: http://www.theremin.info/download-file-37.html.

 

Also, check out the following images of musicians playing a theremin (a.k.a. theremin pilots):

http://www.theremin.info/pics/animatedleon.gif

http://www.gopromusic.com/large_picture.php?id=751&sitetitle=Lew%20Stedman

 

 

Finally, listen to a few short samples of music played on a theremin:

 

For more information on this fascinating instrument, check out the following sites:

http://www.theremin.info/info-47-_What_is_a_Theremin.html

http://en.wikipedia.org/wiki/Theremin

http://www.theremin.info/