Icelandic authorities have raised aviation alert levels to orange after seismic activity around one of the country’s volcanos was detected.

A swarm of earthquakes have been measured by Icelandic experts with the country’s Meteorological Office around the Barðarbunga volcano that could indicate signs of an impending eruption.

Experts raised the alert level from yellow green to yellow on August 16, and then to orange on August 18 after the strongest earthquakes detected around the volcano since 1996.

While officials say there is no sign that magma has been detected moving to the surface, they say that increased activity has prompted them to raise the alert level because in case of a sub-aerial eruption, “an ash plume of potential concerns for aviation will be generated.”

Barðarbunga, one of Iceland’s largest volcanic systems, sits underneath a glacier and has shown signs of activity over the last seven years, but has not erupted since 1910.

Officials say that this activity reduced after the eruption of the Grímsvötn volcano back in 2011, which caused some limited disruption to air travel, although not as severe as that caused by the eruption of the Eyjafjallajökull volcano in 2010. That eruption caused major disruption to travellers across Europe and the U.S.

However, the new wave of activity Barðarbunga has already generated hundreds of earthquakes, with many over a magnitude of three on the Richter scale.

Since the 2010 eruption, Iceland has introduced a five-tier warning system for the aviation community, using colors to display possible activity with grey indicating a quiet volcano while red indicates that an eruption is immient or in progress. The orange alert, issued for Barðarbunga states that the volcano: “shows heightened or escalating unrest with increased potential of eruption.”