Ouvrage Four-à-Chaux, Military museum and Maginot Line fort in Lembach, France

Ouvrage Four-à-Chaux is a military fortress of the Maginot Line in Lembach with underground galleries extending several kilometers beneath the hills overlooking the Sauer valley. The fortification reaches depths of 30 meters below ground level.

Built between 1930 and 1935, the ouvrage was manned in 1938 by the 165th Fortress Infantry Regiment and the 168th Foot Artillery Regiment. It first opened fire on German patrols on 20 May 1940; after Stuka raids on 19 June, the garrison surrendered on 1 July 1940 with military honours. The Wehrmacht then tested explosives, destroying most turrets except turret No. 2.

The fort's name comes from a lime kiln that stood 500 metres away until 1939; in wartime it was known by the code A4. Guided tours, run by the SILE association since 1983, include a museum with a memorial to French and German soldiers killed there. A novel, "Die Tote unter der Maginot-Linie", is set in the bunker and its surroundings.

Visitors should wear comfortable shoes and warm clothing since temperatures stay constant at 14 degrees Celsius (57 degrees Fahrenheit) in the underground passages. The terrain inside requires steady footing and moderate physical fitness.

The munitions entrance lies 24 metres below the troop entrance, and an underground inclined plane—not a rail system—links the two levels. It rises upward from the lower entrance, a feature found otherwise only in some Alpine works such as Le Lavoir. Soldiers pushed carts by hand through the 1.6 km of galleries.