Millions of people around the world have learned CPR on a mannequin known as Resusci Anne. The story of the 19th Century beauty behind the model - or at least, one version of it - will be told at a symposium in London to mark European Restart a Heart Day. But does anyone really know anything about her?

The Lorenzi workshop is a small haven of peace and antiquity in the busy Parisian suburb of Arcueil. And it's the last of its kind. Downstairs the mouleurs, or cast-makers, create figurines, busts and statues, pouring plaster into moulds in much the same way they have since the family business started in the 1870s.

But if you want to be face-to-face with history, pick your way up the dusty wooden stairs to a room above the workshop. It's an unsettling experience. Hanging all around you in the narrow attic are life and death masks of poets and artists, politicians and revolutionaries: Napoleon, Robespierre, Verlaine, Victor Hugo, the robust, impatient face of the living Beethoven and the sallow, diminished features of the composer's death mask.

Yet, surprisingly, of all the visages of the great and the good on display at Lorenzi's, the best-seller is the mask of a young woman. She has a pleasant, attractive face, with the hint of a smile playing on her lips. Her eyes are closed but they look as if they might spring open at any moment. Hers is the one mask that has no name. She's known simply as the Inconnue, the unknown woman of the Seine.

Image caption Busts in the Lorenzi workshop

This is how her story goes. Some time in the late 19th Century, the drowned body of a young woman was recovered from the River Seine. As was customary in those days, her body was put on display at the Paris mortuary, in the hope that someone would recognise and identify her. The pathologist on duty became so entranced by the face of the girl with the enigmatic half-smile that he asked a moulder to take a plaster cast of her face.

Before long the mask began to appear for sale outside the mouleurs' workshops on the Left Bank and soon the young woman's face became a muse for artists, novelists and poets, all eager to weave imagined identities and stories around the mystery woman, this drowned Mona Lisa. Over the years Rilke, Louis Aragon, Man Ray and Vladimir Nabokov successively fell under the Inconnue's spell and at one time no fashionable European drawing room was complete without a mask of the Inconnue on the wall.

Everyone we find in the water, the drowned and suicides, they never look so peaceful Pascal Jacquin of the Paris river police

One of the first stories featuring her was the 1899 novella The Worshipper of the Image by Richard le Gallienne, who portrays the mask as a malevolent force which bewitches and ultimately destroys a young poet. Other authors have been kinder, many of them telling the story of an innocent young woman from the country who comes to Paris, who's seduced by a rich lover and then abandoned when she falls pregnant. With nobody to turn to, she drowns herself in the waters of the Seine, a modern Ophelia. At the mortuary, her beautiful face, now peaceful in death, is preserved forever with a plaster cast.

It was another drowning - or near-drowning - that ensured the Inconnue a place in medical history.

In 1955 Asmund Laerdal saved the life of his young son, Tore, grabbing the boy's lifeless body from the water just in time and clearing his airways. Laerdal at that time was a successful Norwegian toy manufacturer, specialising in making children's dolls and model cars from the new generation of soft plastics. When he was approached to make a training aid for the newly-invented technique of CPR - cardiopulmonary resuscitation, the combination of chest compressions and the kiss of life which can save the life of a patient whose heart has stopped - his son's brush with death a few years earlier made him very receptive.

He developed a torso or whole-body mannequin which simulates an unconscious patient requiring CPR. Asmund wanted his mannequin to have a natural appearance. He also felt that a female doll would seem less threatening to trainees. Remembering a mask on the wall of his grandparents' house many years earlier, he decided that the Inconnue de la Seine would become the face of Resusci Anne. So if you're one of the 300 million people who's been trained in CPR, you've almost certainly had your lips pressed to the Inconnue's.

Through Resusci Anne, students of first aid have for more than 50 years been trying to bring back to life the young woman from the Seine. But was the Inconnue actually dead in the first place? Is that peaceful face really a death mask or was it taken from a live model? When a few years ago I was making a Radio 4 programme about the Inconnue with the novelist Louise Welsh, we decided to show the mask to the men and women who specialise in recovering drowned bodies from the Seine - the Paris river police, known as the Brigade Fluviale.

She looks like she's just asleep and waiting for Prince Charming Pascal Jacquin

Sitting in the wheelhouse of one of the brigade's boats moored at their headquarters near the Pont d'Austerlitz, Chief Brigadier Pascal Jacquin was less than convinced that the girl was dead when the mask was made.

"It's surprising to see such a peaceful face," he said. "Everyone we find in the water, the drowned and suicides, they never look so peaceful. They're swollen, they don't look nice."

Over the centuries, artists and writers may have used the stories of Ophelia and the Lady of Shallot to portray drowning as a romantic and peaceful death but Pascal knows from years of dredging bodies out of the Seine that the truth is very different. Even suicides fight for life at the last moment and their faces betray that struggle. And the process of decomposition starts much more quickly in water too.

This woman, he remarked, "looks like she's just asleep and waiting for Prince Charming to come".

Like so many others before him, Jacquin's imagination had been piqued by the young woman's face and he had already begun to weave a tale of his own around the closed eyes, that secretive little smile and the enigma of the Inconnue.

Other experts we consulted seemed to agree that the Inconnue seemed just too healthy, too vital for the mask to have been taken from a corpse. At the workshop in Arcueil, Michel Lorenzi, the current proprietor, is mystified by people's fascination with the Inconnue.

"This doesn't look to me like the face of a dead person. It's very hard to maintain a smile while a cast is being taken so I think she was a professional, a very good model."

For me the story has two postscripts. The first came a few weeks after the programme was broadcast in 2009. I was visiting Edward Chambre Hardman's photographic studio in Liverpool, the National Trust's perfectly preserved time capsule from the first half of the 20th Century, where anyone who was anyone in Liverpool sat for a portrait with Hardman. Seeing the mask of the Inconnue on the wall of his waiting room, I asked the guide, disingenuously, who the young woman was.

Without hesitating she told me the story of two sisters, identical twins, who'd been born in Liverpool more than a century ago. One of them, she said, had embarked on a love affair with a rich suitor and eloped to Paris, never to be seen again. Many years later the other sister had visited Paris on holiday. Walking down a street she was shocked to see the mask of the drowned Inconnue hanging outside the mouleurs' workshops. She had instantly recognised the girl as her long-lost twin, condemned - or blessed - to remain forever young, while her sister grew old.

I was captivated by the story. Once again, a tapestry of imagination had been woven around the enigma of the Inconnue and in this case it had been given a Liverpool twist.

The other postscript comes from an Oxford-based artist called John Goto. Several years ago he decided to create his own fiction for the Inconnue.

I'd assumed that people would treat it as fictional John Goto

He constructed an apparently factual account of detective work and a trail of clues leading to the discovery of a turn-of-the-century carte de visite in a Buenos Aires junk shop. This and other evidence supposedly proved finally and beyond doubt the identity of the Inconnue - she was as a Hungarian actress named Ewa Lazlo, who was murdered by her lover, Louis Argon. Goto put the story and its "supporting evidence" online and moved on to other projects.

It was intriguing, therefore, to come across details of this week's London symposium for European Restart a Heart Day. During the day a series of speakers share the latest research on improving survival rates after cardiac arrest. But in the evening entertainment is provided by the ensemble Mulberry Hawk. They're putting on a performance which, according to the blurb, "tells the story of Ewa Lazlo who became the inspiration for the face of Resusci Anne, the first CPR training manikin, and the most kissed girl in the world".

So, I thought, Ewa is beginning to take on a life of her own. In fact several websites and blogs are quoting as fact that she's the Inconnue.

Goto admits to feeling a little uncomfortable with his creation. "I'd assumed that people would have a postmodern view and treat it as fictional," he told me. "I really didn't expect them to take it seriously."

But even if Ewa is an object lesson in not believing quite everything you read on the internet, it's unlikely that the story will go much further.

What we love about the Inconnue is the uncertainty, the enigma. As Louise Welsh remarked when we returned from recording our search for this drowned Mona Lisa in Paris, the mask's value lies in its mystery.

"The moment we have a name, a life story, that mystery is dead."

Follow @BBCNewsMagazine on Twitter and on Facebook