The Fortress of Mimoyecques (French pronunciation: [mimɔjɛk]) is the modern name for a Second World War underground military complex built by the forces of Nazi Germany between 1943 and 1944.

Originally codenamed Wiese ("Meadow") or Bauvorhaben 711 ("Construction Project 711"),[2] it is located in the commune of Landrethun-le-Nord in the Pas-de-Calais region of northern France, near the hamlet of Mimoyecques about 20 kilometres (12 mi) from Boulogne-sur-Mer.

The guns would have been able to fire ten dart-like explosive projectiles a minute – 600 rounds every hour – into the British capital, which Winston Churchill later commented would have constituted "the most devastating attack of all".

[5] In May 1943 Albert Speer, the Reich's Minister of Armaments and War Production, informed Adolf Hitler of work that was being carried out to produce a large-calibre gun capable of firing hundreds of shells an hour over long distances.

[10] A study carried out in early 1943 had shown that the optimal location for its deployment would be within a hill with a rock core into which inclined drifts could be tunneled to support the barrels.

[2] The site was identified by a fortification expert, Major Bock of the Festungs-Pionier-Stab 27[notes 1] of the Fifteenth Army LVII Corps based in the Dieppe area.

[11] The area was already heavily militarised; as well as the fortifications of the Atlantic Wall on the cliffs of Cap Gris Nez to the northwest, there was a firing base for at least one[12] conventional Krupp K5 railway gun about 5 kilometres (3.1 mi) to the south in the nearby quarries of Hidrequent-Rinxent.

Owing to technical problems with the gun prototype, the scope of the project was reduced;[2] drifts I and II were abandoned at an early date and only III, IV and V were taken forward.

Along its west side was an unloading platform which gave access to ten cross galleries (numbered 3–13 by the Germans), driven at right angles to the main tunnel at intervals of 24 metres (79 ft).

[16] The construction work was carried out by over 5,000 workers, mostly German engineers drafted in from several companies including Mannesmann, Gute Hoffnungshütte, Krupp and the Vereinigte Stahlwerke, supplemented by 430 miners recruited from the Ruhr and Soviet prisoners of war who were used as slave labourers.

[2] In 1943 French agents reported that the Germans were planning to mount an offensive against the United Kingdom that would involve the use of secret weapons resembling giant mortars sunk in the ground and served by rail links.

[27] The first signs of abnormal activity at Mimoyecques were spotted by analysts at the Allied Central Interpretation Unit in September 1943, when aerial reconnaissance revealed that the Germans were building railway loops leading to the tunnels into the eastern and western sites.

[28] An analyst named André Kenny discovered a series of shafts when he saw from a reconnaissance photograph that a haystack concealing one of them had disintegrated, perhaps through the effects of a gale, revealing the entrance, a windlass and pulley.

Allied intelligence believed at the time that the V-2 rocket had to be launched from tubes or "projectors", so it was assumed that the inclined shafts at Mimoyecques were intended to house such devices.

On 21 March 1944 the British Chiefs of Staff discussed the shortage of intelligence but were told by Reginald Victor Jones, one of the "Crossbow Committee" members, that little information was leaking out because the workforce was predominantly German.

The committee's head, Duncan Sandys, pressed for greater efforts and proposed that the Special Operations Executive be tasked to kidnap a German technician who could be interrogated for information.

The two other drifts were to be reused to house a pair of Krupp K5 artillery pieces, reamed out to a smooth bore with a diameter of 310 millimetres (12 in), which were to use a new type of long-range rocket-propelled shell.

These plans were soon abandoned as Allied ground forces advanced towards Mimoyecques, and on 30 July the Organisation Todt engineers were ordered to end construction work.

[30] The Allies were unaware of this and mounted further attacks on the site as part of the United States Army Air Forces experimental Operation Aphrodite, involving radio-controlled B-24 Liberators packed with explosives.

[27] The Mimoyecques site was never formally abandoned, but German forces left it at the start of September 1944 as the Allies advanced northeast from Normandy towards the Pas de Calais.

"[32] Sandys brought the matter to the attention of Churchill and advised: "Since this installation constitutes a potential threat to London, it would be wise to ensure that it is demolished whilst our forces are still in France.

He concluded at the time, in April 1945, that the intelligence failure had not made much practical difference given the fact that the Germans had failed to develop the HDP into an effective weapon: "there was little warning; [but] there was little danger.

"[7] Following the recommendation that the site should be destroyed, the Royal Engineers stacked ten tons of British 500 lb (230 kg) bombs and captured German plastic explosive in the tunnels at Mimoyecques and detonated them on 9 May.

A subsequent investigation by the British Bombing Research Mission concluded that the entrances had been heavily blocked and that it would be a very difficult and lengthy engineering task to reinstate them.

A complete set of four steel plates,[12] weighing 60 tons, that were intended to protect the entrances to the drifts were bought by the manager of the Hidrequent-Rixent quarries to be cut up for use in rock-crushing machinery.

[40][41] The intercommunality of the Terre des Deux Caps and the authorities in nearby Landrethun set up a partnership to operate the site under the management of the existing museum of La Coupole near Saint-Omer.

The tunnels also house memorials to Joseph P. Kennedy Jr., the other bomber crew members killed during raids on the site, and the forced labourers who lost their lives during construction.