Iceland's Bárðarbunga volcano system was racked by four large earthquakes at the end of October. The rumbles, reported as magnitudes 3.9, 3.2, 4.7, and 4.7, are a sign that pressure is building up in the volcano's magma chamber."Pressure in the system is increasing, that's for sure," says Sara Barsotti, the volcanic hazards coordinator at the Icelandic Meterological Office. This will eventually lead to an eruption, but Barsotti says it’s hard to say when that could happen, or how big the eruption might be.

This volcano could be particularly dangerous if it erupts, since it’s sitting partly beneath Europe’s largest ice cap, Vatnajökull. Volcanoes that erupt under ice can be highly explosive and produce lots of fine ash.