It sounds like the plot of a blockbuster film, but according to scientists, tens of thousands of people in this country face the threat of being poisoned by lethal gas - from volcanoes 600 miles away in Iceland.
Research by a British academic has demonstrated how a volcanic gas cloud emanating from an Icelandic volcano killed 30,000 Britons in a hitherto little-studied environmental disaster two centuries ago.
"People died in such vast numbers because the volcanic cloud exacerbated their respiratory illnesses," said Dr John Grattan, a senior lecturer at the University of Wales, Aberystwyth, who has been studying the impact on Britain of the eruption of Iceland's Laki volcano in 1783.
According to the new research, most victims in Britain died from heart and lung problems caused by the gas and fine dust from the cloud and from associated hot climatic conditions causing additional health problems.
"A similar eruption today would kill up to 100,000 people in this country because we now have a much larger population and a much bigger percentage of it is elderly and therefore more vulnerable," said Dr Grattan.
Iceland poses a particular threat not only because it is relatively near to the UK, but also several of its volcanoes are of a particularly dangerous type because of the vast quantities of atmospheric pollution they can produce.
Iceland has the world's highest number of so-called fissure volcanoes. Unlike ordinary volcanoes, these erupt when vast cracks, sometimes up to 35 miles long, open up in the ground. Icelandic fissure volcanoes can erupt continuously for more than five years.
The 1783 event - an eruption along a 15-mile-long fissure - lasted seven months. What's more, fissure volcanoes don't erupt as explosively as ordinary volcanoes, so gas and ash aren't thrown up into the upper atmosphere but circulate instead lower down, causing severe air pollution over a very wide area. Climatic conditions determine the direction in which the pollution goes.
The major Icelandic fissure eruptions known to historians took place in the 930s, 960, 1227, 1340, 1341, 1477, 1724, 1783 and 1975.
"The 1783 event was in fact only the second largest. The 10th-century eruption was even bigger, lasting from 934 to 939 AD," said Dr Grattan.
Apart from causing widespread respiratory problems, the volcanic cloud of 1783 hit Britain in other ways too. Dr Grattan's new research reveals that initially it helped to raise temperatures and severely damaged vegetation, including crops.
After several months of continuous eruption, sulphur levels in the atmosphere reduced the amount of solar heat reaching the surface and temperatures fell alarmingly.
In a BBC Timewatch documentary to be aired on Friday, Dr Grattan estimates that a further 200,000 people died in France, the Low Countries and northern Italy. In Iceland itself 25 per cent of the population was wiped out.
"Modern society is not prepared for an air pollution event on the 1783 scale. Dealing with it would have immense implications for our lifestyle and economy," said Dr Grattan.