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An incredible cloud phenomenon appearing to stretch thousands of miles across the Earth has sent conspiracy theorists into a frenzy.

The ominous shape resembles a contrail emitted by an aeroplane but is much longer, seemingly reaching from pole to pole.

Online commenters have pitched a host of fanciful ideas as to what may have caused the formation, but experts say it’s simply a rare type of cloud.

It was spotted this week by an eagle-eyed space fan watching a public live feed from the International Space Station.

The mysterious clip was later uploaded to YouTube, baffling conspiracy nuts.

One commenter wrote: “It is a portal for things to travel through unseen/stealth.”

Another quipped: “Looks MAN MADE . . . don’t you think we should have answers for this???????”

But it turns out the strange line has a far simpler explanation.

One weather expert told The Sun the formation was a rare type of cloud.

David Schultz, a professor of synoptic meteorology at the University of Manchester, reckons a low pressure storm is responsible.

“It looks like the back edge of a cloud associated with an extratropical cyclone — i.e. a low-pressure system. It’s unusual, but it is real,” he said.

Satellite images showed a chunk of cloud appeared in the vicinity of the line on March 4, experts at the Met Office explained.

It appears to stretch across a massive portion of the Earth because of a trick of the light.

“Combined with the camera’s perspective, this is making you think there is a very large shadow covering a big chunk of the earth,” a spokesman said.

The perspective trick made the cloud appear “farther and straighter” than the image would have you believe, he added.

Looks like it’s tough luck for conspiracy fans — though perhaps it’s comforting to know a UFO wasn’t blitzing across our skies.

This article originally appeared on The Sun and was reproduced with permission