Last weekend, stargazers in Alaska were treated to an unusual sight; an eerie, pale blue spiral that appeared in the night sky.
It emerged during a display of the Northern Lights, where charged particles from the sun interacted with the Earth's atmosphere to produce bright bands of green.
But despite its UFO or portal-like appearance, the mysterious spiral has a far more mundane explanation.
It was simply excess fuel that had been released from a SpaceX rocket that launched from California about three hours before the spiral appeared.
Similar phenomena occurred over Hawaii in January, and New Zealand last summer, thanks to Elon Musk's space ambitions.
Sometimes rockets have fuel that needs to be jettisoned, said space physicist Dr Don Hampton, a research associate professor at the University of Alaska Fairbanks Geophysical Institute.
'When they do that at high altitudes, that fuel turns into ice,' he said.
'And if it happens to be in the sunlight, when you're in the darkness on the ground, you can see it as a sort of big cloud, and sometimes it's swirly.'
The SpaceX Falcon 9 rocket took off from Vandenberg Space Force Base in California on Friday night.
It was a polar launch, which made it visible over a large swath of Alaska.
The Falcon 9 had 51 spacecraft as its payload, which were being deployed as part of their Transporter-7 mission.
About three hours after launch, on early Saturday morning, a spiral emerged that was caused by water vapor in the rocket's exhaust fluids.
Experts say the spiral shape appears as the upper stage of the Falcon 9 vents unneeded fuel during its long descent into the ocean.
When the rocket fuel is ejected, it freezes and crystallizes in the shape of a spiral, which is then illuminated by the sun.
The timing of the fuel dump was timed correctly for visibility over Alaska. 'And we got that really cool looking spiral thing,' said Dr Hampton.
While not a common sight, Dr Hampton said he's seen such occurrences about three times.
The appearance of the swirly cloud was caught in time-lapse on the Geophysical Institute's all-sky camera and shared widely.
'It created a bit of an Internet storm with that spiral,' said Dr Hampton.
Photographers out for the northern lights show also posted their photos on social media.
One Twitter user said: 'I think Aliens have arrived'
On Facebook, Randell Mailloux from Fairbanks, Anchorage posted a photo and wrote that it was the 'most insane thing [he'd] ever seen by far'.
He said: 'Started out looking like a spiral of northern lights until the center slowly came into view as it flew slowly across from horizon to horizon.
'Then the spiral spread out and covered the whole sky as it dissipated like a ripple in water.'
It is not the first time a SpaceX launch has left observers thinking there might be UFOs present.
In January, another spiral was seen, but this time over Hawaii's Big Island.
It was caught on camera by the Subaru Telescope, located at the summit of Maunakea.
Researchers have said it was from the launch of a military GPS satellite that lifted off earlier on a SpaceX rocket in Florida.
In June last year, a mysterious blue spiral travelling across New Zealand's skies baffled onlookers who thought it had alien origins.
The spiralling plume of gas lit up the sky over Nelson, a city at the tip of New Zealand's south island, and travelled 466 miles (750km) south to Stewart Island.
However, experts later revealed that the phenomenon was caused by man-made space junk in the form of a dying rocket launched by Musk's company.
Last year SpaceX made a record-breaking 61 launches - nearly double its 31 lift-offs from 2021. So far in 2023 it has sent 24 rockets to space.
On Monday, the company's first orbital Starship launch failed due to technical issues, forcing Musk to postpone the mission for 'a few days.'
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