One of the prettiest pieces of music you will ever hear in your life.  Truly inspired from on high.

From Infogalactic:

It was the last of twelve falsobordone Miserere settings composed and chanted at the service since 1514 and is the most popular: at some point, it became forbidden to transcribe the music and it was allowed to be performed only at those particular services, thus adding to the mystery surrounding it. Writing it down or performing it elsewhere was punishable by excommunication.^[1] The setting that escaped from the Vatican is actually a conflation of verses set by Gregorio Allegri around 1638 and Tommaso Bai (also spelled “Baj”; 1650–1718) in 1714.

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.^[1] However, none of them succeeded in capturing the beauty of the Miserereas 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. Less than three months after hearing the song and transcribing it, Mozart had gained fame for the work and was summoned to Rome by Pope Clement XIV, only instead of excommunicating the boy, the Pope showered praises on him for his feat of musical genius and awarded him the Chivalric Order of the Golden Spur.^[2] Some time during his travels, he met the British historian Charles Burney, who obtained the piece from him and took it to London, where it was published in 1771. The work was also transcribed by Felix Mendelssohn in 1831 and Franz Liszt, and various other 18th and 19th century sources survive. Since the lifting of the ban, Allegri’s Miserere has become one of the most popular a cappella choral works now performed.^{[citation needed]}

The original ornamentation that made the work famous were Renaissance techniques that preceded the composition itself, and it was these techniques that were closely guarded by the Vatican. Few written sources (not even Burney’s) showed the ornamentation, and it was this that created the legend of the work’s mystery. However, the Roman priest Pietro Alfieri published an edition in 1840 with the intent of preserving the performance practice of the Sistine choir in the Allegri and Bai compositions, including ornamentation.