The wreck of a German U-boat that sank nearly 100 years ago has been discovered off the southwest coast of Scotland by engineers laying underwater power cables.

Remarkable sonar images show the missing WW I submarine is largely intact off the Galloway coast.

Experts believe the vessel could be the UB-85, a sub sunk by HMS Coreopsis in 1918, according to official records.

Peter Roper, from Scottish Power, said workers found the boat while laying cables on the sea bed to help them survey the sea floor. They generally find boulders and other obstacles like that.

"What we didn't expect to find was a German U-boat," said Roper. "It's probably the most amazing thing I've ever come across in the whole of my construction experience."

Ship attacked by 'sea monster'

Naval folklore suggests the vessel may have been attacked by a "sea monster."

The submarine was caught by the British HMS Coreopsis on April 30, 1918, as it was floating on the surface during the day. The German crew surrendered without resistance.

The beast had 'large eyes, set in a horny sort of skull … with teeth that could be seen glistening in the moonlight. - Captain Krech, German U-Boat captain

When questioned about why his submarine was floating on the surface, the ship's Captain Krech told a wild tale.

He said the sub was recharging its batteries on the surface of the water at night when a "strange beast" rose from the water and attacked them.

Krech described a beast with "large eyes, set in a horny sort of skull … with teeth that could be seen glistening in the moonlight."

The submarine is believed to be the German UB-85, which was caught by the British HMS Coreopsis, pictured, on April 30, 1918. (Scottish Power/International War Museum)

He said every one of his crew began firing at the beast until it dropped back into the sea. But Krech said the sub was so damaged in its battle with the "monster" that it could no longer submerge.

Innes McCartney, a historian and nautical archaeologist who helped identify the wreckage, believes the discovery of the submarine could help solve the mystery of its final hours.

"The Germans lost 178 U-boats in the First World War, 178 at sea, and they are being found now and the number unaccounted for is dropping rapidly and every single time a site like this is found we can reconcile it to the records and slowly build up a much more accurate picture of where they all are and surprisingly, they're not always where we think they might be."

He has a different theory of what may have happened, and it doesn't involve sea monsters.

"The submarine was caught on the surface at night recharging its batteries," he said. "It saw the patrol ship coming, it attempted to do a crash dive to get away and the young officer whose job it was to shut the hatch at the top didn't stop it properly, and when the submarine was underwater it began rapidly flooding from above so they had no option but to just blow all the compressed air they had, bring the submarine to the surface, at which point all they could do was surrender."