The abode

You come across a small valley, barely 400 feet across and maybe 60 feet deep, filled with pines and other northern species of trees. However, among them, you can see the glimmer of polished metal. Coming closer, you can begin to see in the distance a small stone structure, almost perfectly cylindrical and smooth. Above it, at a height of about 10 feet, a dome of polished metal serves as a roof. There are no clear entrance in sight, but someone must have built it. There must be a way to get inside, maybe from underneath?