Two earthquakes measuring over magnitude-5 have shaken Iceland’s Bardarbunga volcano prompting authorities to issue an aviation alert, warning that an ash-emitting eruption may be imminent.

The volcano is located underneath Iceland’s vast Vatnajokull glacier. It has been rattled by thousands of small earthquakes over the past week. Scientists reported a small eruption under the ice on Saturday, but it was not visible on the surface. A no-fly zone over the epicenter has been declared, but airports in Iceland currently remain open.

A 2010 eruption of a volcano in Iceland caused a week of international aviation chaos, with more than 100,000 flights canceled.