Situated under south Iceland’s Mýrdalsjökull glacier, Katla is among the world’s fiercest and most active volcanos. Of all the volcanoes in Iceland, this is the one most feared by the Icelandic people, having erupted roughly twice per century since 930 AD. Its last eruption occurred in 1918, and the next, therefore, is long overdue

Katla’s 1918 eruption was a devastating event that unleashed five times the amount of ash and lava that Eyjafjallajökull—its closest volcanic neighbour—did in the 2010 volcanic blast that grounded air traffic across Europe.

Between October 12 and November 4, 1918, the sky over Iceland turned black and three-quarters of the country became covered in ash. Meanwhile, the country’s southern coastline extended more than 5 km as glacial flood deposits sculpted the vast expanses of black desert sands and obsidian beaches for which the south coast is now famed. Fortunately, there were no casualties.

Over recent years, scientists have measured an increase in seismicity and inflation of the volcano’s caldera, detecting clear warning signs that an eruption is on its way.

No one can pinpoint precisely when Katla will next erupt, but when drastic seismic unrest occurs, all ground traffic across the black sand plains, south of the volcano, will be halted on both sides as a precautionary measure.