By Victoria Gill
Science reporter, BBC News
As scientists and air travellers alike keep a close eye on Iceland's ongoing volcanic eruption, some reports suggest that another, much bigger, volcano could stir in the near future.
Katla is Eyjafjallajokull's more active neighbour, and scientists believe that there may be a link between the two volcanoes.
This link has not been physically proven, explains Magnus Tumi Gudmundsson a geophysicist from the University of Iceland. A circumstantial, historical connection "is putting people's eyes on Katla," he says.
"We know of four Eyjafjallajokull eruptions in the past [dating back to AD 500] and in three out of these four cases, there has been a Katla eruption either at the same time or shortly after.
"By shortly, I mean timescales of months to a year.
"We consider that the probability of Katla erupting in the near future has increased since Eyjafjallajokull went."
Kathryn Goodenough from the British Geological Survey points out that, as yet, there is no physical explanation for this apparent link.
"Scientists don't yet know what the connection is," she says.
"But we know there are fissures running between the two volcanoes. And they're quite close to each other.
"They're also being subjected to the same tectonic forces. So the chances are that if magma can find a pathway to rise beneath one of them, it can find its way to rise beneath the other."
Researchers do know that the two volcanoes have separate magma chambers, but many suspect that these chambers are physically linked in some way, deep beneath the surface of the Earth.
"But this is only speculative," says Dr Goodenough. "We don't have geophysical evidence that makes that clear."
Katla's last eruption was in 1918. It lasted for three weeks and up to a cubic kilometre of material exploded through its vent.
"It's a much more active volcano than Eyjafjallajokull - it has had about 20 eruptions in the last 1,000 years, so it erupts about once every 50 years on average," says Professor Gudmundsson.
The combination of ice and magma makes for an explosive eruption
"At first glance people would say it's now long overdue. But the larger the eruption, the longer the pause (in) time that follows it, and that 1918 eruption was large."
At the moment, there is no seismic activity detectable underneath Katla that would indicate that magma is moving upward underneath it.
Scientists from the Icelandic Meteorological Office are looking at such signals and updating their website regularly with the seismic data that is being produced.
But Dr Goodenough points out that, with Eyjafjallajokull "we only had a few hours warning".
"Seismic monitoring does not necessarily give you advance notice of an eruption."
But it remains a case of watch, wait and look for signs of activity, because it is almost impossible to draw clear conclusions from the historical record, which is simply too short.
While both volcanoes have been repeatedly erupting for hundreds of thousands of years, the earliest eruptions on scientists' records occurred about 8,400 years ago.
Dr Dave McGarvie, a volcanologist from the UK's Open University, who has carried out extensive research in Iceland, explains that in that time Katla has erupted as many as 300 times.
"If we simply look at the historical record, then yes, there appears to be a link [between the two volcanoes]," he says. "But Katla does go off independently, so the link it could be pure co-incidence.
"We just don't have a pattern to get a grip of."
Professor Gudmundsson adds: "We haven't established any physical link [between the volcanoes] - we only have this circumstantial evidence," says . "And we simply don't have enough data to be able to work out what the probability of a Katla eruption is."
Katla is much larger than Eyjafjallajokull, with a magma chamber about 10 times the size.
If and when it does go off, the combination of the magma and the large ice sheet covering the volcano could lead to explosive activity for a long time, says Dr Goodenough.
It is the explosive nature of the current volcanic eruption, which caused an ash plume to be sent high into the atmosphere and affect flights in the UK and Europe.
More worryingly for the people of Iceland, an eruption at Katla would probably cause major flooding. The volcano's ice sheet is 600-700m thick and all of this ice would quickly melt on to the surrounding area, which is primarily agricultural land.
But Professor Gudmundsson says there are "no signs yet" of an impending eruption. "Our eyes are not glued to Katla, we are thinking about the eruption that is happening now."