The term "oktavist" describes a basso profundo singer with an extremely deep range. You'll notice that the singers showcased in this video all sound unbelievably deep, with some making you question whether or not it's humanly possible to produce such low notes. Their voices can go as deep as an octave below the bass staff.
What makes their craft truly impressive, though, is their ability to make audiences hear their voices over an entire choral arrangement without using any amplification. Oktavists spend years training and conditioning their voices for these strenuous performances. While the video only provides examples of Russian oktavists, that doesn't mean that there aren't talented oktavists all around the world.