The cause of Argyll's 'custard lochs' is finally revealed

But the most astonishing scenes came many miles away the next day. "We now have a custard loch," said the Coylet Inn, photographing waves of a thick flaxen slick lapping the shoreline of Loch Eck, the consistency of a sludgy school custard rather than a thin crème anglaise. 

Even further away in Scotland’s northeast, others shared their experiences too. "Everything is covered here in Moray," remarked one person: "You can even taste it."

So what is this, and is it safe? The collective hive mind buzzed into action. The consensus converged on drifts of pollen, as Scotland experiences one of the best spring flowering seasons for many years.

"If it looks a bit dusty, and is floating on the top, then probably pollen," theorized one. "It’s pollen from spruce trees," presented another: "There’s a helluva lot of it about just now (ask your nearest hay fever sufferer) and it’s covering cars, stretches of water, conservatory roofs, etc.." 

"Defo a mix of sweetgale pollen and tree pollen, sitka pollen being more green in colour," proffered a third, "and the pollen that was coming off the myrtles out on the bogland was insane. As soon as you touched it, clouds of yellow sandy pollen went in the air. Should calm down by end of May, and then it’s the weeds’ turn to send out the sneezing fits."

If it is pollen, why are we seeing so much of it now? "Not a lot of rain, so it’s all flying about in the air," offered one theorist. "The zero/light winds have meant it hasn’t blown long-distance," thought another.

A further clue came from a startling video captured near Lochgoilhead by Siobhan Mulholland on Wednesday, showing clouds of yellow "smoke" rising from a tree plantation, which you can watch here.

Siobhan told us: "I would be walking to work, and holiday makers would stop and ask what the pollution was in the water. 

"The weather had been warm, dry, and calm winds for a couple of days. The wee one was in the garden when the wind really picked up. When I seen it starting, I called my man because I thought it was someone starting a fire, and started filming in case anything happened. 

"I’ve been in this house for eight years and never seen anything like it. I suffer really badly from hayfever to and had been suffering for a few day’s before, so could feel it on the air."

Luckily, the Coylet Inn by Loch Eck had some experts staying with them at the time who helped solve the mystery.

A spokesperson said: "We have forestry people stopping with us, and they explained what it was.

"Normally because the pollen is light it gradually falls from the trees in the breeze over a few weeks, but because of the heavy rain and lack of wind it accelerated the pollen dispersal, and washed it rather than blew it, so it ended up in most of the watercourses. 

"We were told it does happen on some occasions but normally very small areas, and it’s very rare to cover such a vast area. It is apparently perfectly harmless."

We asked Forestry and Land Scotland, the Scottish Government agency responsible for managing national forests and land, to see if their experts would comment on the phenomenon.

A spokesperson said it was "very probably" tree pollen from a mix of species including oak, birch, spruce, pine, larch, with ash nearing the end of its pollination season.