Mystery Hill is located in Salem, New Hamphire. Supposedly an ancient Viking site (although other origins have been suggested), the place gives one a fascinating walk through maybes and couldbes. I visited the place several times in my youth, speaking through the secret chamber and laying on the sacrificial rock, all of which convinced me that much of the legend about Mystery Hill is hype. Minus the hype, however, it is still a worthwhile jaunt.
Oh. And Lovecraft evidently visited there and was impressed.