Alessandro Moreschi (November 11, 1858 - April 21, 1922) was the most famous castrato singer of the late 19th century, and the only castrato of the classic bel canto tradition to make solo sound recordings.
A castrato is a man with a singing voice equivalent to that of a soprano, mezzo-soprano, or contralto voice produced either by castration of the singer before puberty or one who, because of an endocrinological condition, never reaches sexual maturity. Castrati should not be confused with eunuchs, who are castrated after puberty and do not share the physical characteristics of someone castrated before puberty.
Castration before puberty (or in its early stages) prevents a boy's larynx from being transformed by the normal physiological events of puberty. As a result, the vocal range of prepubescence (shared by both sexes) is largely retained, and the voice develops into adulthood in a unique way. As the castrato's body grew, his lack of testosterone meant that his epiphyses (bone-joints) did not harden in the normal manner. Thus the limbs of the castrati often grew unusually long, as did the bones of their ribs. This, combined with intensive training, gave them unrivalled lung-power and breath capacity. Operating through small, child-sized vocal cords, their voices were also extraordinarily flexible, and quite different from the equivalent adult female voice, as well as higher vocal ranges of the uncastrated adult male (see soprano, mezzo-soprano, alto, sopranist, countertenor and contralto). Listening to the only surviving recordings of a castrato (see below), one can hear that the lower part of the voice sounds like a "super-high" tenor, with a more falsetto-like upper register above that.
In the 1720s and 1730s, at the height of the craze for these artificially-preserved voices, it has been estimated that upwards of 4000 boys were castrated annually in the service of art.
The Catholic Church's involvement in the castrato phenomenon has long been controversial, and there have recently been calls for it to issue an official apology for its role. As early as 1748, Pope Benedict XIV tried to ban castrati from churches, but such was their popularity at the time that he realised that doing so might result in a drastic decline in church attendance.
Listen: Alessandro Moreschi - Oremus Pro Pontefice (1904)