It's called the Virtual Choir. Eric Whitacre is a contemporary classical composer and conductor and he is the musical genius behind the idea. For each piece they perform (they've done a bunch), Whitacre puts out a call for YouTube submissions from singers around the world. He makes his scores available and asks singers to record their part--soprano, alto, tenor, bass--and then the voices are spliced and edited together to form this incredible, 21st century, international choir.
You must watch and listen.
I keep hitting repeat on this video and every time, I get chills and my eyes well up. This is what art is all about. It's transcendent and fundamentally human and soulful and despite the oceans and continents and languages that separate these people, here they are making heavenly music together. It's astonishing, really.
I hope you love it as much as I do.