Two and a half centuries ago, as he travelled through Italy, the French writer Charles de Brosses found his eye drawn to some of the men. Most, he noted, "become big and fat like capons, their mouths, their rumps, arms, breasts and neck rounded and chubby like women. When you meet them in a gathering you are completely taken aback on hearing these colossal men speak with a tiny, childlike voice."
The French have always made fun of foreigners, but this was not xenophobia. De Brosses's imagination had been caught by the castrati, those gelded stars of the opera stage and Catholic church choirs. It's a subject that continues to fascinate: 2006 is shaping up as the year of the castrato, with an exhibition about to open at London's Handel House Museum, and a BBC4 documentary in the wings.
In an age of televised sex changes, knob-jobs and autopsies, this interest may seem unsurprising. Yet the story of the castrati and the society that created them is still ugly enough to shock.
Imagine, for example, the Melani household in Pistoia, near Florence - where four of the seven sons were castrati. Or the brutality of the Mantuan Duke Carlo III, who had his henchmen abduct a sweet-singing page boy and take him to Venice for castration and musical training in a conservatorio. Imagine the lifelong relief of Gilbert-Louis Duprez - famous operatic tenor of the early 19th century - who only escaped emasculation as a boy when his father forbade his music teachers to take him to Italy for the snip.
And let us remember the many thousands of boys who did go under the knife (up to 4,000 a year, between the age of eight and 13), but whose names are long forgotten. For every superstar castrato who graced Europe's opera houses between about 1600 and 1828 (when Giovanni Battista Velluti became the last eunuch to appear on stage, in a crusader epic by Meyerbeer), there were hundreds whose ordinariness or unpleasantness of voice condemned them to a life in B-grade church choirs, or on the streets as beggars or prostitutes.
And those were the ones who survived the operation. Depending on the executant - at best a skilled surgeon, at worst a back-street barber or abortionist, or the knife-wielders of Norcia, Umbria's pig (and boy) castration capital - there was a very hight mortality rate. Some of these deaths were the result of haemorrhaging or post-operative infection in a pre-antibiotic age. Others came from overdoses of opium, used as a sedative in conjunction with a hot bath and judicious pressure on the carotid arteries.
All those gonads removed, all those pre-pubescent voices preserved across nearly three centuries, until Alessandro Moreschi had the chop just a few years before laws in newly united Italy outlawed castration for musical purposes. The strange, swooping, curdled sound of Moreschi's voice was subsequently captured for posterity on phonograph rolls in the early 1900s. This is fascinating, but by no means easy listening; and the abiding question - how did the 18th century superstars sound? - isn't satisfactorily answered, because, even if Moreschi had been a fine singer once, these recordings from his early 40s show him in less than lovely vocal shape.
By then, the castrato voice type had officially been banned from its last stronghold, the Sistine Chapel Choir. However, Moreschi was allowed to remain singing there until 1913, attesting to the Catholic church's longstanding hypocrisy and ambivalence on the matter. Officially, the act of castration had always been an excommunicable offence, but the appearance of eunuch singers from Spain in Italian church choirs, alongside their renowned falsettist counterparts (broken-voiced adults singing in soprano register), set off a vogue in the late 16th century for a type of voice that had greater longevity and range. So the Church proceeded to tolerate - and, by default, encourage - the harvesting of castrati from the Papal states and beyond.
With the operation officially illegal, common excuses for the boy's unmanned state ranged from his being a natural born eunuch (or "congenital bilateral anorchia"), to a riding accident, an animal bite, castration for the cure of hernia, or the proverbial playground kick where it hurts. In reality, the planned castrations were invariably motivated by the dream of wealth and fame, particularly attractive to families in desperately poor, rustic communities in mid- and southern Italian states.
We know from period accounts that the best singers were mind-bogglingly wonderful. The 18th-century musician Mancini wrote of Farinelli, born 301 years ago and the most celebrated castrato of all: "His voice was thought a marvel, because it was so perfect, so powerful, so sonorous and so rich ... The qualities in which he excelled were the evenness of his voice, the art of swelling its sound, the portamento [sliding between notes], the union of registers, and a surprising agility."
Two hundred years on from the rapid decline of this voice type on the operatic stage, and a century after from Moreschi's eerie bequest to recorded posterity, we are surprisingly well placed to appreciate this strange, disconcerting phenomenon. The high male voice has become less and less unusual, better and more diversely manifested. Countertenor and falsetto voices of rare quality have taken on the castrato repertoire of Handel and his contemporaries alongside their female soprano and contralto counterparts. "Male sopranos" - high-reaching falsettists - have had a go too, often with ear-splitting, leg-crossing results.
We can shuffle on our iPods great countertenors such as Andreas Scholl and David Daniels with the Bee Gees and Michael Jackson. Jeff Buckley's singing of Britten's Corpus Christi Carol is as exquisite and unearthly as the male soprano Randall Wong's renditions of stratospheric castrato arias are astonishing. And in the Darkness's Justin Hawkins wild, ecstatic descants, we hear something almost as outrageous as anything Farinelli might have produced. And involving a lot less suffering.
· Handel and the Castrati is at the Handel House Museum, London W1, from Wednesday until October 1. Details: 020-7495 1685. Castrati will be one of the subjects in BBC4's The Century that Made Us season, screening in the summer.