What we would like to believe

Using 600 separate sonar beams and satellite navigation technology to ensure that none of the loch was missed, the team surveyed the waters said to hide Scotland's legendary tourist attraction but found no trace of the monster.

Previous reported sightings of the beast led to speculation that it might be a plesiosaur, a marine reptile which died out with the dinosaurs.

The team was convinced that such an animal could have survived in the cold waters of Loch Ness, despite the normal preference of marine reptiles for sub-tropical waters.

Looking for the lungs

The researchers looked at the habits of modern marine reptiles, such as crocodiles and leatherback turtles, to try to work out how a plesiosaur might have behaved.

They hoped the instruments aboard their search boat would pick up the air in Nessie's lungs as it reflected a distorted signal back to the sonar sensors.

"We went from shoreline to shoreline, top to bottom on this one, we have covered everything in this loch and we saw no signs of any large living animal in the loch," said Ian Florence, one of the specialists who carried out the survey for the BBC.

His colleague Hugh MacKay added: "We got some good clear data of the loch, steep sided, flat bottomed - nothing unusual I'm afraid.

"There was an anticipation that we would come up with a large sonar anomaly that could have been a monster - but it wasn't to be."

The fence post monster

The BBC team says the only explanation for the persistence of the myth of the monster is that people see what they want to see.

Loch Ness: A beautiful place to visit - even without a monster

Interviewed afterwards, most said they had observed a square object but several drew monster-shaped heads when asked to sketch what they had seen.

The television programme detailing the investigation, Searching For The Loch Ness Monster, was made for BBC One.