Iceland’s most active volcano, the Grímsvötn, could be close to erupting again, experts have shown, claiming there already are multiple indicators. The volcano has already seen 65 eruptions over the past 800 years. The last one occurred in 2011 when it released ash 20 kilometers into the atmosphere.

Local authorities raised the Aviation Color Code from green to yellow after scientists recorded seismic activity indicating magma is swelling in the belly of the volcano. This doesn’t mean an eruption is imminent, but it does show that the Grímsvötn has reached a level of unrest, according to the Icelandic Met Office (IMO).

While an eruption would be unlikely to put anyone in immediate danger due to the remote location of the volcano, it could cause heavy local flooding. The Grímsvötn is buried in thick ice so a blast of heat from the volcano can create vast quantities of meltwater, according to Dave McGarvie, a UK volcano expert.

“Grímsvötn is a peculiar volcano, as it lies almost wholly beneath ice, and the only permanently visible part is an old ridge on its south side which forms the edge of a large crater. And it is along the base of this ridge, under the ice, that most recent eruptions have occurred,” wrote McGarvie in an article in The Conversation.

The ice might cause flooding, but also offers a layer of protection as it will absorb some of the force of the explosion. This means ash will be discharged tens of miles into the air, rather than hundreds, and will disperse more quickly. Still, this might be very bad news for the air travel sector, seeking to recover amid the pandemic.

The volcano is estimated to erupt every five to 10 years, and with nine years since its last eruption, scientists believe it could explode any time now. Usually, an eruption is hard to forecast for scientists, but as Grímsvötn erupts relatively frequently, scientists have been able to pick up on the signs.

First, the base of the volcano begins to expand as it fills with magma. This magma then causes intense heating, which leads to the ice arround the volcano to melt fairly quickly. Both signs have been noted in recent months by local volcano experts, as well as an uptick in earthquakes, another important indicator.

“A high frequency of eruptions at a volcano allows scientists to detect patterns that lead to eruptions (precursors). And if these are repeated each time a volcano erupts then it becomes possible for scientists to be more confident that an eruption is likely to happen in the near future. It’s, however, seldom possible to be precise about the exact day,” wrote McGarvie.

Iceland is home to a large number of volcanoes. While the Grímsvötn is the most active, others have also caused severe damage in the past. The Eyjafjallajökull erupted in 2010 and caused severe chaos in air travel, disrupting around 100,000 flights in April and May, with losses estimated at over $1.3 billion.