And they say it happened over the site of the world’s largest particle collider, which some believe could cause the end of the world.
Footage uploaded online shows the mysterious shape spinning in the skies before a UFO approaches it and disappears into its centre.
Within moments of the strange flying light entering, the alleged portal dissolves into nothingness leaving only blue sky.
“The best or most powerful computers couldn't generate that”Tim Winchester
The clip was released by YouTube channel called Section 51 2.0 – supposedly named because its original account was “censored”.
And the channel’s operators say it was spotted over Geneva, Switzerland, by two visiting American tourists.
But some have disputed the footage, dismissing it as computer generated imagery or perhaps an elaborate hoax.
One Reddit user said: "This video is so compelling, my first reaction is hoax. But that's some great Vfx.
"The clouds dissipate realistically, the tracking is perfect, even during the zoom, color checks out.
"It's hard to say. I think because something this amazing wasn't world news, it's gotta be fake."
While YouTube commenter Tim Winchester says the phenomenon was a sign of the biblical end times.
He wrote: "Not even the best or most powerful computers could generate a visual like that!"
But a quick sweep of news sites seems to show no awareness of the bizarre scene over Geneva predating the video’s upload.
The Large Hadron Collider, built by the European Organisation for Nuclear Research over 10 years, is on the cutting edge of science.
Measuring 27 kilometres in length and entombed in a tunnel beneath the soil, it was designed to shed new light on theoretical particle physics.
But its opening sparked fears that it could cause doomsday phenomena, including microscopic black holes that would grow to consume the earth.