Fifteen vessels carrying researchers and six divers set off from Ostersund, in Jamtland County, central Sweden, equipped with an underwater video camera and echo equipment, hoping to solve the centuries-old mystery.
The town of Ostersund, 370 miles north-west of Stockholm on the banks of Sweden's fifth-largest lake, has been puzzled by reported sightings of a horse-like, or snake-like, creature in the Great Lake.
The sightings, resembling those of the Loch Ness monster, have been reported on 150 occasions by 450 people. They started in 1635 when a local clergyman mentioned the creature in a parish register.
The frequency of sightings rose throughout the 19th century; in 1894 a group from Ostersund set up the "Company to Capture the Great Lake Monster" to trap the animal, using as bait pigs and live calves. King Oscar II, known for his interest in scientific phenomena, became involved and contributed funds. There are no records of success, however.
The project spokesman, Anders Brattgra, said this year's effort was by far the largest and involves a British specialist, Adrian Shine, who has hunted the Loch Ness monster for at least 20 years.
Little is known about the Great Lake of Sweden, which is very large and very deep. Often locals discover rare fish of up to three feet or more in length. "If we have no luck this year we will organise another search, an even bigger one, next year," Mr Brattgra said.
Some reports have suggested a large eel, about 10 feet long and three feet wide, while others report a large snake of up to 46 feet with a small, dog-like head. In 1986, the county administration of Jamtland declared anyone trying to capture, injure or kill the monster could be prosecuted.Reuse content