I had a personal tour of the Gungywamp in CT a few years back by a friend who was a member of the Gungywamp Society. It was an uncanny place with feelings of great antiquity. I was amazed to sit on a boulder and find it was covered with tiny garnets growing out of its surface. I went in the chambers spoken of and pictured near the bottom of this page: Gungywamp in CT.
I remember we were very quiet and respectful as it felt like a very sacred space with a very aware energy. It seems it is becoming more known, but the Gungywamp Society is dedicated to its protection and one still is not suppposed to visit without the permission of the landowner and a member of the society. As far as I know, they have still kept the trail a rough hike with no parking at the bottom available which helps to discourage the hoards.
Here is some interesting info on another of the mysterious sites in New England: Mysterious Megaliths of New England: Mystery Hill.
Also, I remember seeing a docu on PBS one year in the late 80's/early 90's about more and more sites being discovered, along the east coast, which had runic characters carved into the stones, etc. It was very interesting.
My friend who is from the west coast of Wales told us about a city which is under the ocean off of where she grew up. General belief was that it was from Atlantis.