The calmly smiling, beautiful face of this young woman hung in the studios of artists and writers across Europe throughout the late 19th and 20th centuries. She inspired Rilke, Man Ray and Nabokov. And eventually ended up as a first-aid resuscitation model. But who was she? Angelique Chrisafis finds out

I came face to face with Anne only once. She was reclining by a swimming pool in south-east England. Eyes shut, she lay there, slightly smiling, waiting for someone to press their mouth to her cold, thin lips. It all seemed a bit intimidating. Men and women stood around contemplating how to approach her. Resusci Anne - to use her full name - is the world's most famous life-sized doll, and also the most kissed woman in the world. More than 300 million people have embraced her lifelike form to learn mouth-to-mouth resuscitation. "Annie, are you OK? Are you OK, Annie?" students would ask, gently shaking her to practise checking whether a patient is conscious.

In the 50s, Anne's creator, the Norwegian toymaker Asmund Laerdal, deliberately made the doll female, thinking macho men would not want to practise mouth-to-mouth on a dummy of their own gender. But the face Laerdal chose for this plastic woman who would die and be resurrected millions of times is at the centre of a macabre mystery.

The toymaker was a romantic who had been interested in resuscitation since saving his two-year-old son from drowning. He based Anne's face on that of a young unnamed woman who drowned in Paris's river Seine in the 19th century.

The story goes that when the woman's body was fished out of the water at the Quai du Louvre, the Paris pathologist found her so mesmerisingly beautiful that he ordered a moulder to take a death mask of her face. No one ever identified the perfect, blemish-free body. But casts of the beautiful white plaster mask sold across Paris, then across Europe. It hung in the studios of artists and writers from Man Ray to Albert Camus, who described the calm, slightly smiling woman as a "drowned Mona Lisa".

Literary figures including Rainer Maria Rilke, the French surrealists and Russian-born Vladimir Nabokov found a muse in this mysterious French Ophelia, a beautiful young woman it was assumed had taken her own life after she was driven mad by love. Laerdal turned Resusci Anne into a memorial to the unnamed lady of the Seine: it was a way to give her a happy ending over and over again. "Because she has no name and remains an enigma, we can never reach her and taint her ... we project our own dreams on to her," reads the Laerdal company's literature. The St John Ambulance museum in London displays a cast of the unknown woman, in honour of Anne - a face familiar to the more than half a million people in Britain who learn first aid with the association each year.

But one question has persisted along Paris's riversides and beyond: who was this woman and how did she really die? There are many theories. Was she a Hungarian music hall star who had an affair with a married Parisian man, or a young innocent teenager, or a country peasant adrift in the city?

The Seine, with its picturesque bridges and cobbled riverbanks, has always held a morbid fascination for the 12 million people who live beside it and the five million tourists who travel up and down it on tour boats each year. "In the past, legend had it that one would more willingly murder on the banks of the Seine, in the darkness," wrote the French author Pierre Mac Orlan in 1927. "However, the Seine is still the great dumping ground where assassins come to bury their victims more or less dismembered," he added.

Paris's river police, the Brigade Fluviale, whose frogmen pull bodies dead or alive from the river, inspire fascination among Parisians and often appear on television discussing their work. Last year, 50 corpses were retrieved from the Seine's murky water, and 146 people were rescued alive. Around 90 people attempt suicide in the river each year; roughly 70 are saved.

This is nothing new. At the end of the 19th century, Paris's morgue sat perched just behind Notre Dame cathedral, on the edge of the island in the Seine, l'Ile de la Cité. Two-thirds of the corpses dealt with by the morgue would have been fished out of the Seine - suicides, accidental drownings or murders. The morgue attendants would carefully study the dead person's clothes, scars and wounds - often caused after they had hit the water, by a boat or by the hooks used to fish them out. Then the bodies would be displayed on 12 black marble slabs propped up in the morgue window for the public to view and decide whether they recognised any of them.

This macabre showcase became one of the most popular pieces of entertainment in Paris. Locals and tourists peered in at the forsaken departed souls. People of all ages, including children, would visit the famous window of the dead - Émile Zola, in his 1867 novel Thérèse Raquin, described gangs of boys, aged from 12 to 15, "who ran the length of the window, only stopping in front of the female corpses."

Earlier, in the 18th century, families would traditionally float a plank of wood on the river, bearing blessed bread and a candle for the dead. But nothing disturbed and captured the imagination like suicide - especially that of a beautiful young woman who might have taken her life because of a broken heart. When the British director Peter Greenaway made an eerie documentary, Les Morts de la Seine, retelling the tales of 23 drownings between 1795 and 1801, he learned that young women made up the biggest proportion of apparent suicides, appearing to favour drowning, while men opted for hanging. The true stories he recreated included bizarre findings, such as a naked woman in her 70s, dredged from the Seine clasping two leeks in her left hand.

The white plaster cast of the unknown woman first appeared in modellers' shops in Paris at the end of the 1800s. A rare woman's face hanging from the wall among the death and life masks of famous men such as Beethoven and Oliver Cromwell, it was instantly noticeable - and also somehow strange. The woman's hairstyle seemed more like the fashions of the 1860s. And if the cast was really of a drowned woman, how had her flawless face and skin withstood the ravages of water?

The mask's enigmatic smile distracted scepticism. It sold well, bought by artists and writers who hung it in their studios and studies. Soon it stared from the mantelpiece in the living rooms of Paris's bourgeoisie and it became increasingly fashionable in Germany. The German poet Rilke was working as a private secretary to the sculptor Auguste Rodin when he first noticed the mask hanging outside a Parisian plaster moulder's shop in 1905. He described it as "the face of the young woman, which was cast in the morgue, because it was beautiful, because it smiled, smiled so deceptively, as though it knew".

Three years later, Rilke wrote a poem, Washing The Corpse, about laying out the body of an unknown woman:

They washed her neck

And because they knew nothing of

her fate,

They made another one up together,

Washing all the while...

Writers would metaphorically wash the corpse for the next four decades. The first novella to invent a story for the dead woman's mask was published by an English writer, Richard Le Gallienne, in 1900. In The Worshipper Of The Image, a young poet shuts himself away with the mask in an isolated cottage in the woods. He has heard that the man who made the mask fell so much in love with the dead woman that he drowned himself in the Seine. The poet's devotion to the mask, which he calls "le Silencieux", costs him his family and his daughter. All he wants is for the mask to open its eyes, but when it does a moth emerges from her mouth with the face of death between its wings.

La Gallienne's musings set the tone for widespread literary fascination with the drowned Mona Lisa. In France and Germany between the first and second world wars, the cult of the mask took off. She became a symbol of beauty for her era. Writers were attracted by her calm smile. The French philosopher and literary theorist Maurice Blanchot, who had the mask in the main room of his house on the Côte d'Azur, felt she must have died "in a moment of extreme happiness".

On the left bank of the Seine, Hélène Pinet works in the warren of back rooms at Paris's Rodin Museum; she is one of the key sleuths on the case of the woman behind the death mask. "It's a mystery with no leads," she says with a sigh, pulling down from her shelf a book on death masks. "The police archives of the era showed nothing."

It was a 1926 catalogue of death masks that finally gave the woman her name, "L'Inconnue de la Seine" - the unknown woman of the Seine. "From that moment," Pinet says, "the name transcended the mask itself, it was the name that sparked the imagination."

In 2002, the Musée d'Orsay in Paris staged an exhibition of death masks, called Le Dernier Portrait. The Inconnue was featured in it and Pinet set to work on a long essay, trawling the literary and photographic records. "The facts were so scarce," she says, "that every writer could project what they wanted on to that smooth face. Death in water was a very romantic concept. Death, water and woman was a tantalising combination."

In novels, poems and short stories, new lives were invented for the Inconnue. In 1931, the French writer Jules Supervielle traced the thoughts of a 19-year-old woman as she floated, dying, down the Seine. The German actress and novelist Hertha Pauli decided the Inconnue had killed herself after being jilted by a lover. The German playwright Ödön von Horváth imagined her as a seductress who witnesses a robbery and murder in a clockmaker's shop. In 1934 came a bestselling tear-jerker by Reinhold Conrad Muschler, One Unknown. Its central character, Madeleine Lavin, was described by the academic Anja Zeidler as "an innocent young enthusiastic poor provincial orphan of totally pure heart". Somewhat inevitably, young Madeleine makes her way to the bright lights of Paris and falls in love with a British aristocrat, the dashing diplomat Lord Thomas Vernon Bentick. After a romantic tryst, he leaves for Egypt and his fiancée, while Madeleine throws herself into the Seine. Described at the time by the Times Literary Supplement as nauseatingly sentimental, the book sold 175,000 copies in two years and was translated into several languages. In Berlin, two months after the book was published, Nabokov wrote a poem, L'Inconnue de la Seine, telling the white mask:

Amidst pale crowds of drowned

young maidens

You're the palest and sweetest of all

but then, blasting the idea of her innocence, asking if her seducer was perhaps a young cad with "loud tie and gold-capped tooth".

The French writer Louis-Ferdinand Céline, asked by an editor for a picture of himself to accompany a piece he had written, said he was against iconography and instead supplied a picture of the Inconnue. Louis Aragon, one of the founders of the surrealist movement, asked the avant-garde photographer Man Ray to illustrate his 1944 novel Aurélien, in which a 30-year-old Parisian back from the front in the first world war falls in love with a woman the moment he realises she reminds him of the Inconnue de la Seine. The result was 15 photographic "interpretations" of the mask that depicted her lying in bed, aged, and - shockingly - with eyes.

Even in recent decades, one commentator decided the Inconnue was a Russian from St Petersburg called Valérie. An artist claimed she was a music hall performer, Ewa Laszlo, killed by an extortionist who tried to blackmail her married lover - but he later admitted making the story up.

There is one place in Paris where a trace of the truth might be found - at the Lorenzi family model-makers. For more than 140 years, the Lorenzi shop on the left bank attracted the city's best artists and sculptors. It was here that the modeller who took the cast of the Inconnue was said to have been based. Four generations on, Mr Lorenzi's long-suffering ancestors still sell copies of the Inconnue.

Pushed out of their quaint left-bank courtyard by rising rents, the family now work out of an old wood-beamed building in Paris's concrete suburban hinterlands, surrounded by tower blocks. In a battery of plaster busts and death masks, Mozart, Cromwell, Dante and Napoleon hang from the ceiling and there, amid a crowd of poets and French revolutionaries, lies the Inconnue de la Seine, priced at €98 in the catalogue, or slightly more with a glazed finish.

The world of the death mask is a haunt of last breaths and muddled identities. The dashingly handsome young John Keats hangs on the wall, seemingly unfazed by the fact that for decades he was mistakenly catalogued as Baudelaire. The Italian moulder who founded the Lorenzi business brought Keats' mask from London, remembering only that he was a poet and labelling him as the first poet that came to mind - the somewhat uglier Baudelaire. The supposed death mask of the thuggish but handsome French revolutionary Robespierre is, as the Lorenzis point out, actually a life mask - Robespierre blew half his jaw off with a gun before he reached the guillotine.

Amid these hundreds of famous heads, Claire Forestier, a descendant of the original Lorenzi, rummages in a back room for what she calls the Inconnue box. In it, the family have kept all the press cuttings on the mysterious woman of the Seine, but also the dissertations brought to them by students who have investigated the case, claiming her as everything from an Argentinian singer to a French peasant. "This mask crops up everywhere," she says. "Twenty years ago, I was teaching drawing to students at a French beauty school. I walked into a classroom and saw there were 25 lacquered Inconnue faces hanging from the ceiling. They used them to practise make-up on or facial massage."

The Lorenzis sell three or four Inconnues a year, many to writers who want the mask to decorate their homes. Often they are asked who this beautiful woman was. They have no illusions.

"Look at her full, rounded cheeks, her smooth skin," Forestier says. "There is simply no way the cast could have been taken from a corpse. And this is certainly not a drowned woman, fished from the water. It would be impossible to take such a perfect face from a dead woman. Some casts taken from living faces are so clear, so detailed, that when you look at the eyelids you can just see the eyeballs' movement underneath. That's the case with the Inconnue."

The Lorenzis guess that the cast was taken from the face of a mould-maker's daughter or from a young model. Forestier thinks she must have been at most 16 for her skin to be so firm and smooth. She holds up casts of living actors in their 20s and 30s, showing how, even at that age, a plaster mould will show up every crease and indentation of the face.

"I'm sorry. The Inconnue was not dead; more than that I can't say." She sighs. "My only guess is perhaps the girl who sat for the mask drowned herself years later? But I've got no idea and we'll never know."

In the Laerdal company's history of Resusci Anne, the mystery of the Inconnue of the Seine is referred to as all-important. "This seems to be the key to her lasting fascination. Like Leonardo da Vinci's Mona Lisa, and John Everett Millais' Ophelia, the girl from the Seine represents an ideal of beauty and innocence."

If the Inconnue went on to live a long and perhaps happy life, did she have any idea how famous her teenage self had become? Maybe she did know, but chose to stay for ever in the shadows.