The volcano Katla, neighbour to Eyjafjallajökull, has erupted roughly every 40–80 years during the 1,000 years prior to its last eruption in 1918. ‘The eruption is long overdue,’ Dave McGarvie of the Open University in Milton Keynes told New Scientist. ‘There is quite a bit of anxiety in Iceland about the potential size.’

That anxiety is being heightened by the fact that in the past, some of Katla’s eruptions have been triggered by activity in Eyjafjallajökull. Located beneath the Mýrdalsjökull glacier, Katla is one of the largest volcanoes on the island, and its eruptions have a tendency to trigger the sudden release of meltwater from glaciers and ice sheets.

During the 1918 eruption, it was discharging 1.6 million cubic metres of material every second within four to five hours of erupting, and moved so much debris that it added four kilometres to Iceland’s coastline.

‘Eyjafjallajökull is a rather mild volcano, it is not very fierce,’ said Pall Einarsson of Iceland’s Institute of Earth Sciences. ‘Katla, on the other hand, is a rather fierce volcano. It is highly active and it’s dangerous.’