Glen Arbor / Facebook

They look like snowballs that would dent a tractor.

They came on the tides by the hundreds – big, spherical clods of ice whose dirty-brown exteriors made them look like a platoon of Tribbles.

Presumably they're still around, entertaining people around Lake Michigan with their fun appearance and possible usage as nuclear-grade snowballs. Locals call them "ice balls" or "ice boulders," and though they might seem an unnatural, ominous formation to much of the world – something to be pushed back into the water with a bulldozer, perhaps – they've made many appearances in the history of the Great Lakes. Last February, for instance, a flotilla of basketball-sized orbs washed up in the Sleeping Bear Dunes National Lakeshore, allowing Traverse City meteorologist Joe Charlevoix to explain how they're formed:

"The water temperature on the Lake Michigan is just a little bit below freezing, so you get a small piece of ice that forms in the water and as waves move back and forth it adds additional water and freezes in layers. It gets bigger and bigger, and eventually you get big balls of ice, that are pushed to the shore by the wind."

Sometimes the accumulations of ice balls get so dense that they cover vast areas of the lakes like lumpy carpets. They're just one of the stranger winter presences indigenous to the region: Others include pancake ice, icefoots, and (aaahhhh!) the dread "ice volcano."

Some folks in Glen Arbor, a typical northern Michigan town in that it snows uphill, recorded this week's migration of ice balls for their Facebook audience. Have a look:

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