Dan and Dennis Robbins/NWS

These bizarre cylinders self-form in just the right snow and wind conditions.

A hill populated with rotund doughnuts made of snow, with no human footprint in sight—that’s the bizarre spectacle Dan and Dennis Robbins stumbled across after a recent winter storm in Idaho.

These loopy formations are called snow rollers and they’re extremely rare, says the National Weather Service. The snow must be loose and wet, the temperature just right, and the wind gusty enough to get them started … but not too gusty lest they disintegrate. They can form on fields or hills; the ones pictured here near Fairfield, about 60 miles north of Twin Falls, appear to be the slope-dwelling variety.

Here’s a brief explanation of their origins from UCAR:

A small piece of moist, sticky snow may be picked up by the wind and sent rolling, leaving behind a shallow track marking its path. As it rolls, it collects more snow and becomes cylindrical in shape, often with a hole extending lengthwise through the center. The largest snow rollers grow to about one foot (30 centimeters) in diameter.

Idaho seems to be good territory for snow rollers. A man on Facebook recalls encountering them around Twin Falls in the mid-1980s, saying it was the “[o]nly time I ever saw the things.” And these guys formed in 2009 in a Panhandle hayfield—the largest ones measured a whopping two feet tall.

Tim Tevebaugh/NWS

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