There are a lot of big animals in Alaska. Bears and moose are everywhere, and even dinosaurs once roamed the icy landscape. But a video posted to Facebook last week and filmed by a Bureau of Land Management employee had the internet convinced there was another big beast in the state, lurking in the Chena River in Anchorage.

In the video, a 12 to 15 foot long, icy thing looks like it’s slowly swimming upstream in the Chena River. Craig McCaa, who captured the video, added a frame to make the video look like it was recorded on a camcorder rather than his cell phone, sped it up just a bit, and added some sound effects. But otherwise, McCaa says, the video is untouched.

The frigid Alaskan river probably had the right conditions for frazil ice to form on a rope attached to the bridge that McCaa filmed from. Frazil ice is slushy ice that forms in flowing water that’s moving too fast to freeze solid. The icy rope floated to the surface and is moving with the flow of the river.

McCaa’s camera work creates the illusion that it’s swimming upstream. By zooming in, he cuts out any reference points that would give away that the “monster” isn’t actually moving upstream at all.

It may look like you’re seeing a cousin of the Loch Ness Monster, but like other mystery creature sightings, this one has a perfectly plausible explanation.

Photos via Bureau of Land Management - Alaska video screenshot

Kelsey Kennedy is a science journalist from Oregon, now based in New York City. She's written about science, technology, and the environment for Quartz, Undark, and Scienceline.