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

Mighty Katla, with its 10km-wide 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 the 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 have been more than 500 tremors in and around the caldera of Katla just in the last month, which suggests the motion of magma. And that certainly suggests an eruption may be imminent."

The last major eruption occurred in 1918 and caused such a large glacier meltdown that icebergs were swept by the resulting floods into the ocean. The volume of water produced in a 1755 eruption equalled that of the world's largest rivers combined.

Mr Cochran told the BBC what damage an eruption this time might do.

Video produced by Jane O'Brien and Bill McKenna

Photos courtesy of Ford Cochran

Video of Eyjafjallajokull eruption courtesy of Christoph Weber, Volcano Expedition International