Entropy in Concrete

On a hilltop near my home stands a trig point and the capped-off shaft of an old communications bunker, which I believe would have been used in a nuclear war. (see http://en.wikipedia.org/wiki/United_Kingdom_Warning_and_Monitoring_Organisation) Inside the shaft, a ladder leads down to a few rooms with abandoned desks and chairs, bunk beds and wall boards for maps.

It was all accessible ten years ago but now Nature has stepped in to reclaim the hilltop. Our part of Aberdeenshire has the highest concentration of ancient stone circles in the UK and now we are adding a modern version.

What will archaeologists millennia from now make of these concrete lumps when time and change have rendered their original purpose unimaginable?