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Sweden

Gislövshammar

These mysterious circular formations are actually remnants of 19th century gristmills. 

Walking along the shoreline outside of this small village in the south of Sweden will reveal a number of odd, circular formations above and under the water. 

What would probably have been a sign of an ancient alien civilization in any given science fiction movie does, however, have a much more down-to-earth explanation. The limestone rocks in the shoreline were once the perfect material for making millstones for nearby gristmills. The layered stone was easily accessible for mining and the millstones were made to measure before they were sent off to their destinations.

The mining stopped in the last decades of the 19th century, but the mysterious-looking holes from the finished stones can still be seen today, as well as a few stones that were never finished.