Mimoyecques(sp?)
This site was where the forerunner of the Iraqi 'supergun' (the V3) was built during WW2. This also received the attention of Tallboys and Grand Slams, and is well worth a visit. Not that far from Calais, IIRC.
BTW, the U-Boat bunker 'Valentin' is indeed very big - if I can remember how to post them, I will post some pics on here later. Half of it is abandoned, whilst the other half is still in use by the Bundesmarine for storage.