10 times as devastating.
The volcanoes lie under thick glaciers just 16 miles apart in southern Iceland.
And it's estimated that Katla could erupt with 10 times the force of its neighbour if history repeats itself.
The last time Eyjafjallajokull erupted in 1823, Katla followed within months.
Katla erupts around every 60 years but has not done so since 1918. Next time, it will cause vast quantities of ash to be sent into the atmosphere, causing even more disruption to air travel and leading to devastating local floods.
Iceland's volcanoes can cause political unrest. In 1783, the Laki fissure ejected so much ash and toxic gas that it cooled the northern hemisphere for three years, causing crop failures and famines that helped trigger the French Revolution.