Found these at Vandell AFB Denmark, they look like hidden shelters, especially when you look at the left one, you can see an opening..

But what is (was) hidden here, since the rest of the shelters are easily visible, but these aren't and they are quite some distance from the rest...

Vandell AFB was closed in 2003, and was used for communication and navigation perpuses, but it is now being used as a civilian airport...