I just discovered violinist/composer Mari Kimura and her work on subharmonics.
These subharmonic sounds that she creates are absolutely fascinating. Apparently she uses a combination of exact bow pressure, placement, and speed to draw out notes that are lower than the open G string (classically, open G is the lowest note available on the violin). She is able to control this technique, extending the violin's range up to a full octave below open G without modifying the tuning of her instrument.
She talks more about subharmonics and the technique here. She also talks more about her work on her blog here.
