Research into the sinking of the Lusitania involved eerie moments on the U-boat and immersion in the icy waters off Kinsale, for author Diana Preston, she tells Rosita Boland

An immense passenger liner lost at sea almost a century ago, hundreds drowned, the wreck coveted but never recovered. Most people will automatically have the name of White Star liner Titanic passing through their minds when presented with those details. But in 1915, only three years after Titanic hit the infamous iceberg and sank, the Lusitania was torpedoed off the south-west coast of Ireland and sank, with the loss of 1,201 people. Why is it that the Lusitania has never occupied the same place in the public imagination, regenerating itself across the decades as the Titanic has done - and did for decades before James Cameron's film?

"The Lusitania happened in wartime," observes author and historian Diana Preston, whose new book on the liner contains some new archival material. "The Lusitania got subsumed at the time because everyone was preoccupied with war, whereas there were so many stories about the Titanic right from the very beginning. It caught the imagination - the iceberg, the fact it happened out of sight, how it signalled the end of an era. It became a legend."

Preston has spent almost three years researching her book, Wilful Murder: The Sinking of the Lusitania. As titles go, it's blunt and to the point. Preston's research took her to the US, Germany and Denmark, as well as to sources in Britain. In the US, at the National Archives in Maryland, she read much of the material which had been deposited there by relatives of survivors, which helped her to piece together the mosaic of human stories aboard the doomed ship.

Preston's narrative alternates between life on board the liner and on the U-boat which was to torpedo it. The historical and political context of the sinking is also examined, in a clear and unintimidating way. However, it is the chapters recounting the passengers' stories that are the most compelling; just as in the classic of the Titanic genre, Walter Lord's A Night to Remember.

In Germany, Preston worked with the aid of a research assistant. Given that it was Germany that launched the torpedo which sank the Lusitania, was there ever any embarrassment on behalf of archivists about the material they were making accessible? "I think they've been through some sort of catharsis," Preston says. "The awkwardness was probably on my side. But of course, those people were professionals; information is their job, and they have to be detached."

Where Preston did find herself stonewalled was in talking to relatives of those who had been on the U-boat, the U-20, which launched the torpedo. "I talked to the widow of Weisbach [who shot the torpedo]. She was his second wife and much younger; which is why she's still alive. But she was very much keeping to the old party line: that the Lusitania was a legitimate target and that it was carrying armour and troops. She was extremely robust in her views. Polite, but robust," she says.

Irish marine expert John de Courcy Ireland contacted Preston with information related to the U-20. "He told me that part of the submarine was on display in a marine museum in west Jutland, in Denmark."

Preston doesn't write about this in her book, but she went to Jutland to look for the submarine. "I stood inside it, at the place where Weisbach would have stood to shoot the torpedo," she says. "It was compelling and strange; the past made real."

The early 20th century is a period she had already researched in detail for two other books - Robert Falcon Scott and the Race to the South Pole and Besieged in Peking: the Story of the 1900 Boxer Rising. In addition to her interest in the period, an artefact from the Lusitania was one of the triggers for her book. "One of the propellers salvaged from the ship is in Liverpool. I remember looking at it and thinking what a tiny part of something huge it was, like a dinosaur's bone - and wanting to find out more," she says.

While the propeller in Liverpool has been preserved, the fate of another salvaged propeller was not so dignified. "It was melted down and made into golf clubs," says Preston with incredulity. "Everywhere I go, I keep asking people if they have ever come across one of these golf clubs, but so far nobody has."

The Lusitania sank on May 7th 1915, 10 miles off the Old Head of Kinsale, en route from New York to Liverpool. All aboard were aware of a possible risk from torpedoes (sufficiently so to perform life-jacket drills) once they entered European waters. It was only three years since the Titanic had gone down, so the Atlantic passengers who came after them put their faith in the talismans of life-jackets and lifeboats, believing that if there were enough of each for everyone, they would be all right, come what may. In the event, the ship sank too fast for lifeboats to be launched safely, and many of those that were launched were dragged under in the massive undertow.

Among the passengers was Sir Hugh Lane. It was understood he was carrying art treasures with him, reputed to be by Monet, Rembrandt and Rubens and insured for $4 million. They were placed in lead containers in the hold. To this day, nobody knows for certain which paintings Lane had with him, but they were almost certainly significant ones.

In the course of her research, while looking at files in London's Public Record Office, Preston discovered a document confirming what had long been suspected: that the U-20 had only fired one torpedo. Yet the ship sank in a mere 18 minutes, a fact Britain blamed on her being hit by two torpedoes, and Germany claimed was caused by illicit arms in the hold of a passenger ship. Passengers also reported hearing a second explosion. The document Preston found was a message from the U-boat's captain, intercepted the British, saying he had only fired one torpedo.

The blame and counter-blame continued for decades, but politics and naval strategy were of little concern to the relatives of those lost at sea or, indeed, to those Irish people who stood on the Old Head of Kinsale in 1915 and witnessed with disbelief the disappearance of the huge ship.

Preston came to Ireland when her research was near completion. "I wanted to wait until places like Cobh and the Old Head really meant something," she says. She literally immersed herself in her research, eerily standing in the cold water of the Atlantic in an effort to understand why so many of the passengers who were rescued later perished from hypothermia en route to Cobh.

The location of the wreck was pinpointed in the 1930s. Ever since, it has been coveted by treasure hunters and professional ghouls, and ownership of the wreck has changed hands several times. The purser's safe, where gold and jewellery were deposited for safe-keeping, has never been recovered. There were reports from one dive of metal tubes, which could possibly be the Lane canvases, being spotted on the sea floor. Reports of this sighting, suggests Preston, could be the reason Ireland, in whose jurisdiction the wreck lies, has passed legislation forbidding the removal of any artefacts from the site.

"If there are certain things lying scattered on the sea-bed, I think it would be appropriate to take some of them and put them in a museum where everyone can see them," Preston says, choosing her words carefully. "But it would need to be done within the next few years, as the wreck is collapsing in on itself."

 Wilful Murder: The Sinking of the Lusitania by Diana Preston is published by Doubleday (£18.99 sterling)