MARINE scientists have solved a centuries-old Scottish mystery, by uncovering what causes the Corryvreckan whirlpool.
Known as the speckled cauldron and feared as a lethal trap for careless sailors, the whirlpool between Jura and Scarba is the third-largest in the world.
It was long assumed to have been caused by strong Atlantic currents hitting a pinnacle of rock sticking up from the seabed, forcing water from the deeper eastern side up against the shallower western side.
Oceanographers from the Scottish Association for Marine Science (Sams), however, have now mapped the seabed for the first time using high-resolution multibeam echo-sounder technology and have been very surprised to find no pinnacle.
Instead, the huge maelstrom is probably caused by a steep-sided buttress of rock protruding from the Scarba shore.
Dr John Howe, a geologist from Sams, said he and his team were delighted to have shed light on a natural feature of such scientific and cultural importance.
He said: "For the first time in many years, one of Scotland's most turbulent stretches of water has been surveyed to an international standard. This is extremely good news for the safety of people at sea."
He added: "It's not just the end of a mystery, it's opened up new ones. The aim now is to try and understand how the flow of water has affected the seabed. That will help us try to understand and predict tides and the flow of water, and the transport of pollutant and sediment."
The survey revealed that, due to the powerful currents surging between the two islands, the seabed in the area has been scoured clean.
Dr Howe added: "Any mud and sand on the seabed has been completely swept away. Beyond the whirlpool the seabed has thus been swept up to form areas of underwater dunes made of very coarse sand that move with the currents."
The area was investigated this summer as part of a survey of the entire Firth of Lorn by scientists from Sams along with colleagues from the UK Hydrographic Office.
The work forms part of the Ireland, Northern Ireland and Scotland Hydrographic Survey project (INIS Hydro).
Echo-sounder technology emits hundreds of sound beams per second and collects information from the echoes to construct detailed seabed maps.
The vortex of tidal currents at the Corryvreckan can run at over eight knots and it is considered highly dangerous by sailors.
In 1947, the author George Orwell, in Jura to finish Nineteen Eighty Four, came close to drowning with his young nieces and nephews after sailing too close to the pool.
He lost control of his craft in the smaller eddies that surround it and the boat lost its motor. The party managed to row to the safety of a rocky island, where the boat promptly capsized and they were later rescued by a passing lobsterman.
Corryvreckan has been steeped in folklore for centuries and remains a major draw for tourists.
One legend features a Scandinavian prince, Breakan, who tried to win a local princess by holding his boat fast for three days in the whirlpool, using ropes of hemp, wool and hair provided by Norwegian maidens.
He failed on the third night and is sucked into the whirlpool after the hair rope breaks – one of the women was not as pure as she claimed, says the legend.