IT is a mystery that has captivated people around the world since it slithered into the public imagination in the 1930s. But scientists carrying out a groundbreaking study may just have pinpointed the true nature of the Loch Ness monster.

Having combed the loch for samples of environmental DNA, researchers believe the most plausible explanation for Nessie: a giant eel.

The research, led by professor Neil Gemmell from the University of Otago in New Zealand, involved 250 water samples taken from the edges, centre and very depths of Britain’s largest body of fresh water by volume.

The DNA from each sample was isolated and sequenced and then compared to global DNA databases in a bid to create a comprehensive picture of life in the loch.

The findings quickly debunked one of the most popular theories: that the Loch Ness monster could be a reptile, or population of reptiles, which survived from the time of the dinosaurs, such as a plesiosaur.

Other theories suggest Nessie may be a giant catfish, a giant sturgeon, an eel or even a Greenland shark, which can live for up to 500 years.

The only possibility not ruled out by the research was that of a giant eel – perhaps explaining Nessie’s looped shape in the popular imagination.

Gemmell said: “There is a very significant amount of eel DNA. Eels are very plentiful in Loch Ness, with eel DNA found at pretty much every location sampled – there are a lot of them.

“So, are they giant eels? Well, our data doesn’t reveal their size, but the sheer quantity of the material says we can’t discount the possibility that there may be giant eels in Loch Ness.”

He added: “Divers have claimed they’ve seen eels that are as thick as their legs in the loch, whether they’re exaggerating or not, I don’t know, but there is a possibility that there are very large eels present in the loch.

“Whether they are as big as around 4m (13ft) as some of these sightings suggest – well, as a geneticist I think about mutations and natural variation a lot, and while an eel that big would be well outside the normal range, it seems not impossible that something could grow to such unusual size.”

The professor said that further research was needed to test the theory, but added: “Based on our data, giant eels remain a plausible idea.”

Another finding from the research was the high levels of DNA from land-based species in the loch, including humans, dogs and farm animals such as sheep and cattle. DNA from wild animals such as deer, badgers, foxes, rabbits, voles and multiple bird species was also found and could aid studies of biological diversity.

Adrian Shine, leader of the Loch Ness Project, which collects scientific data in the area, said: “Gemmell’s environmental DNA survey of Loch Ness has furnished us, at a stroke, with species lists, to compare with those we have compiled over the last 40 years with net and microscope.

“This powerful yet elegant technique has also brought a new maturity to the popular debate about what much larger creatures might sometimes be seen here.”

A documentary about the hunt for Nessie’s DNA is due to air on the Discovery Channel on September 15.