La Coupole is a Second World War museum built into a vast underground bunker complex dug into the chalk hills south of Saint-Omer between 1943 and 1944. The Germans intended it as the main launch site for V-2 ballistic missiles aimed at London and south-east England — a facility that would have allowed them to fire rockets at a rate of dozens per day. RAF bombing raids caused enough structural damage that it was never completed, but the structure itself is one of the most extraordinary pieces of wartime engineering in France.
The museum inside covers both the V-weapons programme (V-1 flying bombs and V-2 rockets) and the broader history of the Second World War in northern France and the occupied territories. A real V-2 rocket is displayed vertically in the main chamber. The underground galleries — long concrete corridors lit in cold fluorescence under the massive chalk dome — create an atmosphere unlike any conventional museum. Essential viewing if you have any interest in WWII history.