Keighley-born deep sea diving expert Graham Jessop has helped to locate the first German submarine to sink a Royal Navy cruiser.

Mr Jessop, head of salvage at RMS Titanic, said the latest project could have solved a mystery which has puzzled historians for almost 90 years.

The company's hi-tech exploration vessel, SV Explorer, has located U-boat U21 in the North Sea using sophisticated sonar equipment.

The submarine - and its captain Otto Hersing - entered the history books when it sank the British Light Cruiser HMS Pathfinder on September 5, 1914, off St Abb's Head, Northumbria.

It was the first submarine casualty of the First World War and only the second time in history that a ship had been sunk by a submarine. The first was a confederate submarine in the American Civil War.

U21 went on to cause havoc in the Mediterranean, said Mr Jessop, who led one of the first dives on the Titanic, the passenger liner which sank off Newfoundland in 1912 with the loss of about 1,500 lives.

"The U-boat played a crucial role in the battle of Gallipoli when she sank the Triumph," said Mr Jessop, who lives in France.

"Australian troops on the beaches were robbed of covering fire and hundreds were killed as a result.

"But it was the way the submarine eventually sank which has always been suspect."

Official accounts of U21's last journey claim it foundered in the North Sea while on route to surrender to the British Grand Fleet at Lowestoft in 1919.

"But there is some suspicion it was scuttled by the crew.

"Our cameras saw that the hatches were open so it's unlikely to have foundered and indeed could have been scuttled," he added.

He said the search for U21 was on behalf of Econova TV Productions, of Nova Scotia, which is filming for a National Geographic International TV series called Sea Hunters.

The series is based on the book, The Sea Hunters, by author and deep sea diver Clive Cussler.

The book recounts some of Mr Cussler's dramatic exploits in wreck hunting over the past 20 years.

U21 was one of his early targets but he was unable to locate it.

Although the submarine is only 45 metres underwater, it was the sophisticated sonar equipment on SV Explorer which helped to locate her, Mr Jessop said.

The vessel is now moving on to explore the liner Carpathia, the ship which rushed to the stricken Titanic and rescued 700 people.

The wreck was discovered last year and Mr Jessop will lead the expedition.

e-mail: clive.white

@bradford.newsquest.co.uk