"Overtone singing is a voice technique where one person sings two notes at the same time," explains German musician Anna-Maria Hefele.
Hefele is an 'open throat' singer - and her amazing ability has seen this video go viral. Not least because the sound of a vocalist holding a low note (the 'fundamental') while simultaneously singing a scale of high notes (the 'overtone scale') is utterly bizarre, and somewhat Twilight Zone-ey.
She also teaches this technique - and if you're interested in learning it (and fancy a trip to Austria), she's shortly running a workshop in Salzburg, 25-26 October. You know what to do, polyphonic overtone singing fans!