It was a lovely late summer afternoon at a beach in southern England. Sun, surf and not a cloud in the sky — until the strange "chemical haze" drifted in off the sea, that is. It was at that point the beachgoers found their eyes streaming tears and their throats growing sore, their gag reflex triggering as some began to vomit.

Then, the professionals in hazmat suits showed up.

More than a day later, authorities still aren't exactly sure what happened to people at Birling Gap beach on Sunday.

But here are the details we do know: The stretch of shoreline was shrouded in the "plume of gas" for a span of about three hours, according to local Sussex police, during which time authorities advised people living nearby to keep their doors closed and windows shut — and "to move away from the Birling Gap area in particular."

At least one witness didn't get that advisory, though.

"We got home, threw open the windows to air the house and quickly developed stingy eyes," Jonathan Hill told the BBC. "At one point I couldn't see for about five minutes."

"Whatever it was, it smelled like burnt plastic," said another witness.

By the time the mist had dissipated, roughly 150 people had been treated for their symptoms at Eastbourne General District Hospital. Police believe hundreds more were affected by the mist, though in general, they say, the effects were minor.

And that's about where the certainties end.

In fact, the vague and vaguely menacing assessment offered initially by local Sussex police is as accurate now as when it was first published: "This seems to have been caused by an unknown haze coming in from the sea, but the source has not yet been established."

Of course, that has not prevented speculation from swirling.

Many armchair chemists on social media posited the cloud was actually chlorine gas — though police note the local fire department "has advised that it is 'extremely unlikely' that it was chlorine." Likewise, police say it's also "very unlikely" the source lay in northern France, as an East Sussex official suggested earlier.

So, what could it have been?

Beats everyone, it seems. Sussex police have assured residents that "investigations continue into nature of gas & source," though they did not specify which agencies were conducting those investigations.

In the meantime, authorities say they have stood down, saying they believe it was an isolated incident and that it's once more safe to open windows and doors.

Still, one beachgoer remained unconvinced.

"My only concern now is I want to know what it is," she told The Guardian. "If they don't know what it is or where it came from, how can they say it's safe?"

Copyright 2021 NPR. To see more, visit https://www.npr.org.