Spem in Alium is a 40-part motet: forty separate vocal lines (eight choirs of five lines each). Sometimes the voices imitate each other, interweaving so seamlessly that it's impossible to pick one line and follow it. Sometimes they move together in vast and glorious chords, vast but not burdensome; it's as if the music is poised en pointe in a moment that seems endless, suspended in space and time, before spinning into the dance once more.
If you are a singer, and you ever get a chance to sing in it, grab that chance with both hands. To sing it is to stand in the middle of a work of creation or evolution: to see galaxies form and grow and blossom into slow-motion supernovas, finally stabilising in a rich, harmonious universe. In the beginning is the word, and the word is hope, and everything grows from that: one single note, two notes meeting in a bare yet perfect fifth, and from then on an exponential unfurling into complexity and majesty.
And to listen? To listen is to stand on the sidelines and to realise, with a growing sense of wonder, that the sidelines are also the middle; that you are the point at which the interweaving melodies converge; that you are both a part of the creation and its purpose; that you are the still point of the turning world.