UFOS shaped like mysterious black triangles or diamonds have been repeatedly spotted flying over Britain and Europe.
The objects were intercepted by fighter jets in Belgium, tracked by a NATO radar travelling at incredible speeds, and one potentially even crashed at an RAF base.
Shapes in the sky appear to have menaced the airspace over Northern Europe for the better part of a decade – and there has still be no answers as to what they may have been.
“It is still a genuine mystery – these triangle sightings in the 80s and 90s are significant, but still unexplained, ” former Ministry of Defence UFO investigator Nick Pope told The Sun Online.
Was it all an elaborate hoax? An example of mass panic? Alien spacecraft? Repeated misidentification of conventional planes? Or perhaps top secret US technology?
Britain always denied it was aware of any experimental US aircraft operating in its airspace – but declassified documents show Whitehall feared it was being kept in the dark by the Americans.
Mr Pope added: “The 80s and the 90s marked the height of a worldwide wave of sightings of triangular and diamond-shaped UFOs.
The civil servant, who joined the MoD in 1992 and worked in Whitehall until 2004, said the situation reached “near-hysteria” behind closed doors with many being “confused and conflicted” over the reports.
He said the similarities between many of the sightings had been “spooky” – with mystery aircraft able to accelerate at speeds “many times faster than conventional jets”.
“And this was coming to us from people in the air force and pilots – not just wide-eyed witnessed,” the investigator added.
And one of the most compelling incidents involving these triangles or diamonds is the so-called “Calvine Photograph“.
The photo shows an angular object appearing to be hanging in the sky – with a Harrier fighter jet also visible – before witnesses claim it jetted into the sky at a high rate of speed.
Mr Pope first revealed the existence of the photo in his 1996 book Open Skies, Closed Minds – but he says he cannot confirm the veracity of the newly released image which was released this month after being found in the possession of a former RAF press officer.