Anna-Maria Hefele from am-oberton.de performs what is called polyphonic overtone singing. Watch this video and hear how someone could sing multiple notes at the same time. I never thought this could be possible and would even jokingly challenge good singers to sing and harmonize with themselves if they really were good singers.
This, however, absolutely blew my mind. If you listen with earphones you would appreciate the music better. The best part is at 3:25 where Anna-Maria sings fundamental notes and overtone in the opposite direction.
Polyphonic overtone singing is also known as throat singing.