Grímsvötn - the Icelandic volcano that grounded thousands of flights across Europe in 2011 - could erupt again in the near future, according to experts.
Scientists monitoring seismic activity around the volcano have recorded an increase in movement in recent weeks, meaning an eruption could be imminent.
The Icelandic Meteorological Office (IMO) has said "multiple datasets" indicate Grímsvötn has reached "a level of unrest comparable to that observed prior to historic eruptions."
Here’s everything you need to know.
What is Grímsvötn?
Grímsvötn is Iceland's most active volcano, and stands at 1,725 metres tall at its highest point.
The huge ash clouds created by the volcano's last eruption in 2011 affected flights across continental Europe.
Hundreds of planes were grounded, and the eruption even affected US President Barack Obama's state visit to Ireland, cutting his visit a day short with the threat of further air travel disruption.
When will it erupt again?
Concerns around the imminent eruption of Grímsvötn were first raised in June 2020, after scientists reported high levels of sulfur dioxide.
The IMO then issued a warning that an eruption might take place in the coming weeks or months.
Grímsvötn is almost entirely covered by thick ice, and the IMO warned that a glacial flood as a result of melting ice could trigger an eruption.
On 30 September, the IMO raised Iceland's Aviation Colour Code from green to yellow, though it did note that "this does not mean that an eruption is imminent."
"The conditions at the volcano may change at any given time and the volcano may return to normal background conditions without further escalation,” the IMO said.
Would another eruption halt flights?
Although another eruption could take place in the near future, it is not expected to be as disruptive as previous instances.
Dave McGarvie, a Volcanologist from Lancaster University, said "the next eruption should be a small one", adding that "the word 'should' is important here."
Writing for The Conversation, McGarvie said that if the next eruption is indeed triggered by melting ice, the resulting ash would get "wet and sticky", therefore falling from the sky "relatively quickly."
"Ash clouds therefore only travel a few tens of kilometres from the eruption site,” explained McGarvie.
"This is a good scenario for Icelanders and also for air travel, as it prevents the formation of substantial ash clouds that could drift around and close off airspace."