The Miserere is written for two choirs, one of five and one of four voices, and is an example of Renaissance polyphony surviving to the present day. One of the choirs sings a simple version of the original Miserere chant; the other, spatially separated, sings an ornamented "commentary" on this...Whenever I hear Mozart stories, I reflexly think of him as Tom Hulce's Amadeus character.
Three authorized copies of the work were distributed prior to 1770 – to the Holy Roman Emperor Leopold I, to the King of Portugal, and to Padre (Giovanni Battista) Martini. However, none of them succeeded in capturing the beauty of the Miserere as performed annually in the Sistine Chapel. According to the popular story (backed up by family letters), the fourteen-year-old Mozart was visiting Rome, when he first heard the piece during the Wednesday service. Later that day, he wrote it down entirely from memory, returning to the Chapel that Friday to make minor corrections. Some time during his travels, he met the British historian Dr Charles Burney, who obtained the piece from him and took it to London, where it was published in 1771. Once the piece was published, the ban was lifted; Mozart was summoned to Rome by the Pope, only instead of excommunicating the boy, the Pope showered praises on him for his feat of musical genius.
Text credit Wikipedia, with a hat tip to beasterne.