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Some Boulder residents on Wednesday were perplexed by a twisting cloud formation above the foothills.

Several people posted photos of the odd phenomenon on social media.

Marshall Shepherd, an atmospheric sciences professor, posited that the sight could have been a rare helix cloud, which requires either contrails or thin clouds to be spun by a twisting cylinder of air.

The National Weather Service said the wind disturbance was likely a mountain wave. Chuck Haight posted on Twitter that he thought the cloud was a natural formation, as he did not witness any planes in the area while it was forming.

Of course, others on social media weighed in with not-as-scientific explanations, including chem trails, aliens and unicorn farts.

This is not the first time Boulder County has been enthralled by a strange cloud formation. In 2017, a spaceship-shaped group of lenticular clouds made its way across the county and onto social media.

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