The mystery in the forest

Suddenly it was there, in front of me. I was going down through steep scree and thick vegetation when it emerged. A kind of enclosure, formed by natural positioned rocks and stones put down by some people. And an opening in the east.
But what was it?
Made by kids?
Not likely if it`s done lately. Don`t think kids of today would bother to go up in a thick forest to carry stones.
An old animaltrap? No, it didn`t seem to be so old, either.
Maybe a hideout for moosehunters? But are mooses capable to walk around in this scree?
Well, I hope someone can tell me what it might be.

Latest news:
Commentators on the Norwegian blog suggests that it is a machinegun-nest. Either from the second world war or from the local Homeguard.
Probably.