They say frozen natural gas produced by deep sea bacteria has built up over thousands of years to provide the huge energy reserve.

Dr Ben Clennel, from the Earth sciences department at Leeds University, has made a special study of these methane ice "gas hydrates".

Dr Ben Clennel: "There is twice as much energy"

Speaking at the British Association Science Festival being held at Cardiff University, he said: "If you add together the energy content of all the conventional fossil fuels - that's oil, gas and coal - and you compare it with the energy content in natural gas hydrates, you find that there is about twice as much energy potentially available in natural gas hydrates than there is for all the fossil fuel resources that have been exploited in the past. And we think we can exploit that in the future."

Gas hydrates are effectively frozen mixtures of ice and water sitting in mud a few metres below the ocean floor in a layer a few hundred metres thick.

They are found in both low and high latitudes, and there are believed to be concentrated fields of the ice off the Shetland Isles.

But exploiting gas hydrates presents a major challenge because they are difficult to reach.

Dr Clennel: "It's a big technological challenge"

"Gas hydrates are only found within the sediments under the deep oceans at water depths of between 500 and 3,000 metres. And that's very inaccessible," said Dr Clennel.

"The only way we can tap into them is by dangling a very long drill pipe from a ship and then drilling through the sediments into the gas hydrate layer and then to the layer of gas that's tapped underneath."

Dr Clennel said Japan and the United States are already investing millions of dollars investigating the technology.

He said scientists hope to drill straight through the frozen layer to reach the easily extractable gas bubbles, but warned there are dangers as well as benefits associated with gas hydrates.

If global warming raised the temperature of the sea to an extent where the frozen methane melted, huge amounts of gas would be released into the atmosphere.

Methane is a greenhouse gas 10-times more powerful than carbon dioxide, and its release from the sea could greatly accelerate global warming.

Dr Clennel said that because the hydrates were deposited in layers on the edges off continents, if they became unstable major submarine landslides could be triggered.

Around 7,000 years ago, unstable gas hydrates caused a huge landslide off Norway, which in turn produced a tidal wave swamping much of the Shetlands.