Aberdeen Coastguard were alerted at just before 4pm on 25 May 2001 after what looked like an oil slick formed half a mile off Occumster, Caithness.

Local creel fishermen David Scadding and George Carter set out to investigate.

Mr. Scadding said, "It was a large black slick which must have been 100 yards long by 20 yards wide. As we approached it, I thought it was odd as there was no sign of any oily sheen.

"We were then in the middle of the largest swarm of flies you could imagine – there was billions and billions of them. It was the strangest experience I’ve had in my life."

The bow of the boat was soon caked in the flies as they were washed up by the swell of the sea. The pair put a bucket over the side to take a sample of the insects, most of which appeared dead. The sample has been passed on to coastguards, who are consulting experts to try and identify the species and account for how so many had come together at once.

Aberdeen Coastguard is as mystified as anyone about the sighting.

"At the moment we’re completely baffled," a spokesperson said. "It sounds like a major natural phenomenon and we are keen to learn more about it."

Mr. Kevin Colcomb, a senior scientist with the Maritime and Coastguard Agency, is arranging to have the sample examined by entomologists.