Hundreds of metres under one of Iceland's largest glaciers there are signs of a looming volcanic eruption that could be one of the most powerful the country has seen in almost a century.

Mighty Katla, with its 10km (6.2 mile) crater, has the potential to cause catastrophic flooding as it melts the frozen surface of its caldera and sends billions of gallons of water surging through Iceland's east coast and into the Atlantic Ocean.

"There has been a great deal of seismic activity," says Ford Cochran, the National Geographic's expert on Iceland.

There were more than 500 tremors in and around the caldera of Katla just in October, which suggests the motion of magma.

"And that certainly suggests an eruption may be imminent."

Scientists in Iceland have been closely monitoring the area since 9 July, when there appears to have been some sort of disturbance that may have been a small eruption.