By Dan Walker
BBC Two's Timewatch
A little over 200 years ago, the eruption of a volcano in Iceland sent a huge toxic cloud across Western Europe. It was the greatest natural disaster to hit modern Britain, killing many thousands - but it has been almost forgotten by history.
"Such multitudes are indisposed by fevers in this country that farmers have difficulty gathering their harvest, the labourers having been almost every day carried out of the field incapable of work and many die."
So wrote Hertfordshire poet William Cowper in the summer of 1783.
Across the country, newspapers reported the presence of a thick smog, and a dull sun, "coloured like it has been soaked in blood".
The cloud first reached Britain on the 22 June 1783. In his Naturalist's Journal, Gilbert White reported: "The peculiar haze or smoky fog that prevailed in this island and even beyond its limits was a most extraordinary appearance, unlike anything known within the memory of man."
The killer cloud lasted weeks, if not months, and engulfed much of Western Europe - as thousands of kilometres away in Iceland, the volcano Laki continued to erupt.
Millions of tonnes of toxic gas were carried by the prevailing winds across Scandinavia and eventually to Britain.
The cloud contained sulphur dioxide and sulphuric acid which attacked the lungs of its victims, choking and killing men and women, rich and poor alike.
The events are better documented in Iceland where up to a third of the population died. Yet, incredibly, the British tragedy wrought by Laki has been largely forgotten.
Evidence now brought together by BBC Two's Timewatch makes clear the extent of the disaster.
Panic and fear were widespread - as was death. But just how many died, no-one knew until recently.
Dr John Grattan of Aberystwyth University, Wales, has spent a decade scrutinising hundreds of local parish records looking for evidence of Laki's deadly effect.
"In Maulden (in Bedfordshire) the normal number of people who might be expected to have died in the summer would be about four or five - and in the summer of 1783 seventeen people die here.
"In nearby Cranfield, 23 people die in the summer and usually they'd see about six. And in Ampthill, it's 11 and usually it's about five. So parish by parish, these numbers add up considerably."
Dr Grattan's research revealed a similar pattern across the county, and across much of eastern and central England.
From the fives and tens in each parish, Laki's death toll increases into the hundreds, then thousands, then tens of thousands.
In total, he estimates Laki's killer cloud took the lives of 23,000 British men and women, making it the greatest natural disaster in modern British history. France and other countries were similarly hit.
And it could happen again. Iceland has 18 volcanoes that have been active in recent centuries, the greatest concentration anywhere on the planet.
"There will be another one," says leading vulcanologist Professor Stephen Self, of the Open University, who has studied the Laki eruption.
"It's difficult to predict what size it will be, but there will be future events like this from Iceland.
"Ash clouds, gas clouds, sulphuric acid clouds from Iceland could sweep across Britain again."
Timewatch: Killer Cloud is on BBC Two on Friday 19 January at 2100 GMT