Greenham Common : A Place of Mystery. What is it?

Right next to the main road entrance to the common car park, near the control tower, is a short bit of road, which leads nowhere at one end, and to this place at the other. I cannot guess what it is or was, and it is securely fenced and locked. The only possibility I can think of is that it was somehow electrical in nature. The last picture says it all.

Google Earth shows the place, right next to the entrance to the car park

We stand at the Going Nowhere end of that bit of road

As we walk down the road we begin to see the fenced place of mystery

Grey thing looks more like a pigpen than anything else!

Pretty secure

Ah! I thought so - I am not alone in not knowing what it is! (OK, I admit, I also know how to PhotoShop!)