A mysterious chemical cloud drifted over the south coast of Great Britain recently, and so far, nobody knows what it was.

On Aug. 27, beachgoers at Birling Gap, a popular destination about 60 miles south of London, were enjoying a lovely day — until a haze engulfed the coastline, local police reported in a press release. People started getting sick — stinging eyes, difficulty breathing, vomiting — and ambulances were called.

Police advised people to stay indoors and keep their doors and windows shut, though one resident told the BBC he didn't hear the warning. "We got home, threw open the windows to air the house and quickly developed stingy eyes. At one point I couldn't see for about five minutes," said Jonathan Hill, adding that it smelled like burning plastic.

More than 150 people were treated at hospitals for their injuries, which authorities said were mostly minor, and police believe hundreds more were affected by the "plume of gas."

The video below was taken by a man on the U.K. beach as the unknown haze settled in:

It lingered for about three hours and has since gone away. But where did it come from, and what was it?

The above tweet and other social media posts suggest chlorine, but East Sussex Fire and Rescue Service said that is "extremely unlikely." The BBC reports that chemical clouds have drifted to the U.K. from industrial plants in northern France in the past, but Sussex Police said that is also "very unlikely" due to weather patterns in the area.

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The suspicious mist at #birlinggap yesterday...

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"Neither the gas nor its source have been established, but agencies are continuing to investigate and have not ruled out either on-shore or off-shore locations," Sussex Police said in a statement.

For now, it all remains a mystery ...

Angela Nelson ( @bostonangela ) is an exhausted mom of two young daughters and two old cats, and a Pulitzer Prize-winning digital editor with more than 15 years of experience delivering news and information to audiences worldwide.

Mysterious 'chemical cloud' wafts over British coast, sickening hundreds
What was that chemical haze that engulfed a U.K. shoreline south of London, and where did it come from?