The Throat Singers of Tuva

Testing the limits of vocal ingenuity, throat-singers can create sounds unlike anything in ordinary speech and song--carrying two musical lines simultaneously, say, or harmonizing with a waterfall

via The Throat Singers of Tuva: Scientific American.

How is it done? One person commenting on YouTube says "You make an L shape in your mouth, the tip of the tongue being the top of the L, and the back of your tongue lifts and flattens out to change the pitches."