Earthquake rumbles under the Vatnajökull glacier in Iceland could be signs of an impending eruption by the country's biggest volcano.
Bárðarbunga, which stands 2,009 metres (6,591 ft) above sea level, is one of a number of volcanoes that geologists are carefully monitoring after a spate of recent earthquake activity - indicating that the pressure in the volcano is increasing.
"The reason for the earthquakes in this place is that the volcano Bárðarbunga is inflating, i.e. the pressure of magma in the magma chamber is increasing," volcano expert Páll Einarsson at the University of Iceland told the Daily Star.
"The volcano is clearly preparing for its next eruption, that may happen in the next few years. The earthquakes last week are just the symptoms of this process, they do not cause the volcano to erupt."
Bárðarbunga has been rumbling, he said, since February 2015 - when the volcano's last eruption, beginning August 2014, ended. Prior to that event, the volcano had been causing earthquakes with increasing frequency since 2007 - and the eruption itself was preceded by a swarm of 1,600 eruptions within 48 hours.
The 2014-2015 eruption of Bárðarbunga was relatively light in consequences compared to the earlier 2010 eruption of the smaller Eyjafjallajökull. A subglacial stratovolcano, Eyjafjallajökull's modest-sized eruption caused unusual and disproportionate havoc.
The heat of the volcano melted the ice cap, which caused floods. Then it spewed ash several kilometres into the atmosphere - where it was carried thousands of kilometres over Europe by the jet stream above.
So thick and far-reaching was the ash that air travel all over Europe was disrupted for weeks after the eruption.
Bárðarbunga is also a subglacial stratovolcano, and although a repeat of Eyjafjallajökull's havoc is possible, it's unlikely - as demonstrated by the volcano's 2014-15 eruption.
Disaster expert Simon Day of University College London told the Daily Star that Bárðarbunga "is statistically unlikely to do so."
There are other rumblings in Iceland too - earlier this year, Einarsson told the Iceland Monitor that four volcanoes were on the path to eruption sometime in the next few years.
But the world has also been waiting on an eruption from Katla for decades.
The Iceland Meteorological Office is not concerned yet. All four volcanoes - in fact, most of the country's volcanoes - are marked green under the aviation colour code map. This denotes that the volcanoes are "in a normal, non-eruptive state."