I saw Werner Herzog's documentary about Antarctica, Encounters at the End of the World, over the weekend, which is currently playing at Cinema 21. (And oh my god, go see it. It's incredible.) Herzog uses an eclectic selection of music to soundtrack the movie, including Bulgarian choirs and the unearthly underwater calls of Weddell seals (which sound like early Moog synth noises):

Towards the end of the film, Herzog uses a piece of choral music that features a Russian basso profundo voice. The vocal is deeper and lower than anything I've ever heard. Check out this YouTube clip which features an example of this type of singing:

Now that's low.