At the fringe of the Eperlecques forest you will find the gigantic bunker. Built by slave labour for the Nazis to build and fire their V1 and V2 reprisal weapons.
It was only about a third completed when the Allied bombers found it and it was heavily bombed, the "tallboy" bombs used were able to smash through the 5 metre thick concrete and render the site useless. The Germans then start what became quite normal for weapons such as these and used mobile launch vehicles.
From the picture you will see the sheer scale of the building. The lad, Jay, in the front of it is about 5ft 6" tall; you can make him out on the full size photo.