The polyphonic German singer Anna Maria Hefele can sing two different notes at the same time, in which the mid and high range keys harmonise to uncanny effect. It's known in German as Obertongesang ('Overtone Singing') - she sounds like a living Theremin. (There's a wonderful clip of Louis Theremin, demonstrating his invention here.) Do watch and listen to both.
I was reminded of the glass harmonica, invented by Benjamin Franklin (yes, THE Benjamin Franklin) in 1761, a sophisticated take on the old party trick of making a wine glass 'sing' by rubbing the rim with one's fingertip. Do look this up.
At one time the glass harmonica was much in vogue, especially in Europe, and pieces were composed for it by Mozart, Handel, Beethoven and Richard Strauss. Its use peaked in the 18th century for several reasons - it wasn't loud enough to fill a concert hall, being more suited to the smaller aristocratic music rooms of a less egalitarian age; composers began increasingly to score for modern stringed instruments, brass, woodwinds, and percussion; the instruments themselves were terribly fragile and often shattered in transit. There were also rumours that exposure to its melancholic sound could make both musicians and audiences go mad, as reported by the German musicologist Friedrich Rochlitz in the Allgemeine Musikalische Zeitung:
The harmonica excessively stimulates the nerves, plunges the player into a nagging depression and hence into a dark and melancholy mood that is apt method for slow self-annihilation. If you are suffering from any kind of nervous disorder, you should not play it; if you are not yet ill you should not play it; if you are feeling melancholy you should not play it.
(The phrase 'apt method for slow self-annihilation"should appear on cigarette packets rather than the boorishly capitalised SMOKING KILLS.)
But back to Anna Maria Hefele. I see her demonstration (to which I link above) has attracted over five million viewers which I suppose makes me what marketing people call a 'late adopter'. I know almost nothing about her. She's been performing this type of singing since 2005 (so I guess she's in her thirties), comes from Grafing bei München, a town in the Upper Bavarian district of Ebersberg about 30 km from Munich.
She would, I think, be more at home in theatre than opera - but how to use such a skill?