Didn't know singers could do this sort of thing

I recently got a recording of Vivaldi's opera Bajazet, which is really quite good. It's a bit different structurally from later opera like Mozart or Beethoven, which makes it interesting. And there are some arias where the singers are doing stuff with their voices that I didn't know could be done! And I had no idea that anyone ever wrote music like that! If you're familiar with Vivaldi's violin concertos, you'll remember that there are lots of places where the violin part alternates between a high and a low string in 16th-note fashion. Well, in Bajazet the singers do the equivalent, alternating between high and low notes really quickly. Amazing...