America's Stonehenge

America's Stonehenge is a collection of stone structures whose origin is unknown (the most common theories include earlier-than-though Europeans or an advanced Native American culture), but that dates to several thousand years ago, making it possibly the oldest known man-made structure in the US. Like Britain's Stonehenge, the site is an accurate astronomical calendar, and most structures line up with solar and lunar events. It is located in Salem, New Hampshire on top of Mystery Hill, surrounded by typically serene New Hampshire woods.

I highly recommend visiting the site at some point, it's a neat experience.
Information about it can be found on the website (http://www.stonehengeusa.com), along with a map that helps identify the locations of these photos.