No, not a little midget dolmen.
Old well, publicly accessible property somewhere in Connecticut or Massachusetts. The flash on the camera is a handy way to judge if there is actually water near the surface, without actually sticking one’s hand in or leaning in too far, the latter tends to make people nervous. Now how deep that water is? Anywhere from 5 to 25 feet, with an average of 10.
Acair Fearann 
I am so jealous! You find such wonderful things.
That is actually in an area where the public is permitted…it just would be a rather long hike!
Fascinating! How I would love to come upon such a sight.