Thanks to Stephen King, monsters and mist go hand-in-hand. But on Sunday, in East Sussex, England, the monster was the mist itself. A strange haze descended on beaches along the southern coast, leaving authorities puzzled and dozens of people in need of medical treatment for stinging eyes, sore throats, and vomiting, The Independent reported.

With temperatures at 77 degrees, beachgoers had thronged to the pebbly shores between Eastbourne and Birling Gap, south of London, for the penultimate weekend before school starts. But the cloud, which allegedly smelled of something between chlorine and burning plastic, ended the fun.

Those affected took to Twitter to air their concerns: Kyle Crickmore tweeted about his “streaming” eyes, while Dan Sankey described “some weird mist, burning everyone’s eyes which led to the beach and cliff being evacuated.” The smog also provoked some tongue-in-cheek commentary on Eastbourne’s local slogan: “Breathe it in.”

But the source of the cloud remains unknown, The Guardian reported. Despite whispers that it might have come an industrial chemical operation across the channel in France, a source close to the investigation has suggested that it might have been vented from a passing ship. Even gas monitoring equipment has so far failed to conclusively identify either the source or composition of the toxic haze. In a release, East Sussex Fire and Rescue Service explained that, so far, they have no clear leads in their investigation, though they report that it is unlikely to have been chlorine gas. “This makes it more difficult to discover where it came from,” they said.

Whatever it was, said Bob Jefferey of the Royal National Lifeboat Institution, it seems to have mostly dispersed. “It hung around and didn’t move yesterday because there was no wind.” The hospital has since downgraded the threat, and people have returned to the beaches.