Mystery Hill: America's Stonehenge?
About 40 miles north of the city of Boston, and about 25 miles inland from the Atlantic Ocean, is what appears to be the greatest, and perhaps oldest, megalithic enigma of North America. Mystery Hill, also known as "America's Stonehenge", is a site that has puzzled archaeologists for almost a century.
Running across the 30 acres of hillside are a series of low walls, cave-like primitive buildings, and tunnels that are spread about with, according to one archaeologist, "gigantic confusion and childish disorder, deep cunning and rude naively."
While the hill is compared to the English Stonehenge circle, it is, at first glance, physically quite different. Stonehenge is located on a plain, not a hill, and is arranged neatly as a series of concentric circles, horseshoes and squares. Mystery Hill seems a jumble in comparison. The stones involved in Stonehenge are larger, up to 45 tons. The stones at Mystery Hill are smaller (the largest is about 11 tons) and the construction less intricate.

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http://unmuseum.mus.pa.us/mysthill.htm