Jonathan Downes would not look out of place on an X-Files set.

He is an expert on the supernatural and boasts vampires, giant snakes and Westcountry big cats among his specialist subjects.

And tomorrow, Jonathan will be travelling to Lancashire to get to the bottom of more strange happenings in the depths of the mysterious lake known as Martin Mere.

There have been frequent sightings of a giant creature in the lake's waters, and many visitors have witnessed swans being attacked.

Now Jonathan, 42, from Exwick, and other members of the Exeter-based Fortean Society, are determined to track down the demon of the deep.

In doing so they will be investigating treacherous and uncharted waters.

The lake, near Ormskirk, is steeped in mythology. Among its compendium of spine-chilling stories are tales of dragons, unexplained ancient human mutilation and even a resident mermaid.

Martin Mere is also believed to be the resting place of the most famous of the kinights of King Arthur's Round Table, Sir Lancelot.

However, Jonathan and his team of Richard Freeman, 32, Graham Ingliss, 45, and John Fuller, 40, are hoping for a more scientific explanation.

He said: "Although sometimes I have to come to terms with the fact that I am dealing with things outside conventional science, this time I am not going up there expecting to find a giant monster. Instead, we are expecting to find a wells catfish, a species which should not be found in Britain."

It is thought that a wells catfish might have been introduced into the lake by Francis Buckland, of the Acclimatisation Society, during the 19th Century.

The society, which was made up of a group of scientists, experimented in introducing exotic animals into Britain in the 1850s.

John added: "It is a lovely idea that the fish in Martin Mere is the same one that was introduced by Francis Buckland all those years ago.

"This is certainly possible, as they grow very slowly, and some can live for hundreds of years.

"They can also reach a weight of about 300kg and get up to 16 feet long.

"If it is a wells catfish, it will almost certainly be a British record for the biggest fish in Britain."

Pat Wisnieski, centre manager at Martin Mere, believes that Jonathan may have found the answer to the mystery:

"We know that over the last century there have been illegal attempts to introduce the Wells catfish into British waters," he said

"The giant catfish could conceivably survive in the murky, de-oxygenated water for years and grow to an extremely large size.

"This would seem to explain all the incidents which have occurred of wild swans being dragged under the water.

"On two occasions earlier this year something in the water has caused over 1,500 wild wintering swans to completely disappear from the lake.

"However, all the swans we have seen being attacked by the monster have survived."

The team of specialists will be using a range of hi-tech equipment to find the swan-snatcher.

"We will be using electronic fish-finder sonar equipment, fluorescent rope and bamboo stakes to get a 3-D contour map of the lake," Jonathan said.

"People may have seen us practising this on the river down by the Mill on the Exe.

"By doing this we will be hoping to find where the fish is lurking, and then we will buy mackerel heads and pull them along in a pillow behind our boat to bring it to the surface. We are also getting floating pond pellets for the other fish in the lake to feed on, to attract this big predator to come up to feed."

To clear up the mystery for good, John will be using military-style infra-red cameras and night-sights to get photographic evidence of the 'Monster of the Mere'.

In recent years, the Fortean Society, which is 10 years-old, has taken its detective work to countries far and wide.

As Britain's expert Fortean Zoologists, they have travelled to Puerto Rico in Central America to investigate El Chupacabra, a vampire-like animal targeting goats.

The team has also hunted dangerous giant snakes in the rainforests of North Thailand and researched Nevada's giant Thunderbird - the mythical bird icon of native Americans.

John is also sure of the explanation behind the mystery of the big cats on the Westcountry moors. "I have seen them and they are definitely panthers which have been released or have escaped from zoos," he said.