Three centuries ago in parts of North America, a strange event turned morning to night. It remains wreathed in mystery - so what caused the Dark Day?
Halfway through the morning the sky turns yellow. Animals run for cover and darkness descends, causing people to light candles and start to pray. By lunchtime night has fallen. Is it the end of the world?
The Dark Day, as it's become known, took place on May 19, 1780 in New England and parts of eastern Canada. For the past 232 years historians and scientists have argued over the origins of this strange event.
Today there are many theories. Was it the result of volcanic eruption, fire, meteor strike - or something more sinister?
When the makers of Doctor Who this week asked fans of the show to send in their suggestions, they received a wide range of theories both plausible and Tardis-related.
With little scientific knowledge amongst the populace in 1780, people would have been afraid. Some lawmakers in Connecticut believed it was the day of judgement. The sense that a decisive moment was afoot would have been bolstered by the fact that during the preceding days, the sun and moon glowed red.
Historian Mike Dash says the north-east corner of the US was a deeply Protestant society with a profound interest in "guilt, sin and redemption". Dash, who wrote about the paranormal in his book Borderlands, says that faced with sudden darkness, people would look for biblical precedents.
"There are some verses in Matthew that might have led them to believe that this is the second coming of Christ. At the time, natural events - even birds fighting in the sky - were a sign of God's intentions. The Dark Day would have seemed like a warning to Man."
So what might explain 1780's Dark Day?
The Met Office points out that thick cloud can drop low enough to turn on automatic street lights and require cars to use their lights. But it's unlikely this alone would be enough to cause a Dark Day.
A solar eclipse can be ruled out as there is a record of when these occur - and they only last for a matter of minutes.
The eruption of the volcano Eyjafjallajokull in 2010 caused enough ash to enter the atmosphere to ground flights across northern Europe.
Thomas Choularton, professor of atmospheric science at the University of Manchester, says volcanic ash clouds often cause "yellow days". Eruptions at Mount St Helens in Washington State have lowered light levels in recent decades, he adds.
And yet there is no record of volcanic activity in 1780, he says, making a huge ash cloud an unlikely explanation. A meteorite is equally unlikely, although "you can't rule it out completely", Prof Choularton says.
The answer to the puzzle can be found in the trees, many scientists believe.
Academics at the University of Missouri's Department of Forestry analysed tree trunks inland from New England, where westerly prevailing winds would originate. They found signs of fire-scarred rings in tree trunks dating back to that period.
It is also known that there was a drought there in 1780 making fire more likely, says Dr Will Blake, associate professor of geography at Plymouth University.
But could a forest fire cause such a change in light? "I've witnessed minor fires in Australia where you get a very eerie light. The bigger the fire, the darker it's going to get." Fog is common on the east coast. The mix of fog and soot from the forest fire would combine to make darkness descend, Dr Blake argues.
Eyewitness accounts in New England support the forest fire hypothesis. Soot was spotted in the rivers. And Jeremy Belknap of Boston wrote in a letter that the air had the "smell of a malt-house or a coal-kiln".
William Corliss, the physicist and chronicler of unexplained events, found 46 accounts of dark days around the world between 1091 and 1971.
Nowadays people can call upon scientific knowledge, satellite pictures and the media for reassurance. But Dark Days have continued to unsettle people until surprisingly recently.
A Dark Day in a similar part of North America to 1780's occurred in 1950. It was caused by forest fires in Alberta and prompted alarm and confusion, says David Phillips, senior climatologist at Environment Canada.
"If you'd woken up at noon you'd have believed it was midnight. People thought it was nuclear attack or a solar eclipse."
Whatever the cause in 1780, the geography must have exacerbated the fear, says Dash. Settlements tended to go little more than 200 miles inland. In essence, European settlers were living on the edge of a vast unknown continent.
"When it goes dark for them, there's no guarantee it is ever going to get light again. In those days it would be quite natural to think it was the Second Coming," Dash says. When dawn arrived, it is likely that prayers of thanks were said across the previously benighted land.
Here is a selection of readers' fanciful explanations of Dark Day.