Experts are warning that an eruption could be imminent at an even more powerful Icelandic volcano than the one that paralysed air traffic last year.
Seismologists are nervously watching rumblings beneath Katla which could spew an ash cloud dwarfing the 2010 eruption that cost airlines two billion dollars (£1.27 billion) and drove home how vulnerable modern society is to the whims of nature.
Brooding over rugged moss-covered hills on Iceland's southern edge, Katla is a much bigger beast than the nearby Eyjafjallajokul volcano, which blasted ash all over Europe for several weeks in an eruption that local scientist Pall Einarsson describes nonetheless as "small".
Named after an evil troll, Katla has a larger magma chamber than Eyjafjallajokul's.
Its last major eruption in 1918 continued more than a month, turning day into night, starving crops of sunlight and killing off some livestock.
The eruption melted some of the ice-sheet covering Katla, flooding surrounding farmlands with a torrent of water that some accounts have said measured as wide as the Amazon.
Now, clusters of small earthquakes are being detected around Katla, which means an eruption could be imminent, seismologists say. The earthquakes have been growing in strength, too.
After a long period of magnitude three tremors, a magnitude four quake was detected last week.
"It is definitely showing signs of restlessness," said Mr Einarsson, a professor of geophysics at the University of Iceland.
Teams of seismologists and geologists at the university are tracking the spike in seismic activity and working with disaster officials to prepare communities near Katla like Vik, a small town of some 300 people that is flanked by black sand beaches.
Civil defence authorities have been holding regular meetings with scientists. Disaster officials have also drafted an evacuation plan and set aside temporary housing, but many fear they may have less than an hour to evacuate once the volcano erupts.
Iceland sits on a large volcanic hot spot in the Atlantic's mid-oceanic ridge. Eruptions, common throughout Iceland's history, are often triggered by seismic activity when the Earth's plates move and magma from deep underground pushes its way to the surface.
The longer pressure builds up, the more catastrophic an eruption can be. Records show that Katla usually has a large eruption twice a century. Since its last eruption was almost exactly 93 years ago, it is long overdue for another, seismologists say.
Icelanders are getting nervous as they mark the anniversary of Katla's last blast.
"We've been getting calls recently from people concerned that Katla is about to erupt because it erupted ... in 1918 on October 12," said Einar Kjartansson, a geophysicist at the Icelandic Meteorological Office.
"As scientists we don't see that much of a correlation in the date but there is most definitely increased activity. The question is whether it calms down after this or whether there is an eruption."
Of Iceland's more than 22 volcanoes, seven are active and four are particularly active - including Katla and Hekla.
Although it does not pose the same flood risk as Katla because it's not situated beneath an icecap, Hekla is one of Iceland's most active volcanoes and sits in the path of most international flight patterns.
Like Katla, Hekla is also overdue for a large eruption and could produce a disruptive and dangerous ash cloud that, in addition to disrupting air travel, could lower overall temperatures across continents by blocking out sunlight for days or weeks.