LA CROSSE, WI (CNN) -- It looks like something out of a science fiction movie!

What you see in the video is swarms of mayflies.

They hatched over the weekend in La Crosse, Wisconsin, covering street lights, buildings, stairs, nearly anything in their path.

Mayflies are harmless and they hatch several times a year. But the National Weather Service says this weekend's swarms were so big they could be seen on radar.

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