The Katla volcano, hidden beneath the ice cap of Mýrdalsjökull glacier in Iceland, has historically erupted violently once every 40-80 years. In-as-much as it's last such eruption took place one hundred years ago, in 1918, Katla's next eruption is long overdue.
Katla Volcano
© Fréttablaðið
AN ICE CAULDRON IN MÝRDALSJÖKULL Geothermal activity in the volcano’s caldera melts the glacier, creating cauldrons in the ice.
An eruption in Katla would dwarf the 2010 Eyjafjallajökull eruption, scientists have warned.

A new study by Icelandic and British geologists showed that Katla is emitting enormous quantities of CO2 - at least 20 kilotons of CO2 every day. Only two volcanoes worldwide are known to emit more CO2, Evgenia Ilyinskaya a volcanologist with the University of Leeds told the Icelandic National Broadcasting Service RÚV.

These enormous CO2 emissions confirm significant activity in the volcano, Evgenia told RÚV: "There must also be a magma build up to release this quantity of gas."
"It is well known from other volcanoes," said Evgenia, "that CO2 emissions increase weeks or years ahead of eruptions. This is a clear sign we need to keep a close eye on Katla.... there is something going on."
https://icelandmag.is/article/new-study-confirms-monster-volcano-katla-charging-eruption

The largest glacial flood in Iceland's history occurred in the beginning of the Katla eruption in 1918. Tales of this flood are terrifying. Although it was a thick mixture of meltwater, volcanic ash, and ice, it advanced so fast that one could only escape its path on horseback. It covered hundreds of square kilometers; today such a flood would destroy the Iceland ring road and many important facilities in the south of the country.