Ouvrage La Salmagne, Military fortification in Vieux-Reng, France.

Ouvrage La Salmagne is an underground fortress in Vieux-Reng consisting of two combat blocks linked by tunnels. These passages housed barracks and supply facilities below the first block to protect soldiers stationed there.

The original fort was built between 1884 and 1895 as part of the Séré de Rivières system around Maubeuge, and it surrendered to German forces on 5 September 1914 during the siege. The Maginot ouvrage was approved in 1934 and built inside the old walls for 6.2 million francs. On 22 May 1940, German 150 mm guns silenced Block 2, and the garrison surrendered at 20:30.

Volunteers of Amifort, the Association des Amis de la Forteresse du Secteur Fortifié de Maubeuge, staff the seasonal reopening and keep the site, the only fully visitable Maginot fort turned museum between Dunkirk and the Ardennes. The fort stands within the earthworks of the 1884-1895 Séré de Rivières fort of the same name, so a visit covers two generations of French fortification.

The site is best explored on foot, with stairs and narrow passages leading through underground sections. Visitors should wear appropriate shoes and bring a jacket, as the tunnels remain cool and damp year-round.

The fort's Maginot-era garrison in 1940 was 97 men and 3 officers of the 84th Fortress Infantry Regiment under Captain Brichard. The Germans took the position on 22 May 1940 after 15 cm guns fired into Block 2's embrasures; the mixed embrasure was punched through, the 47 mm anti-tank gun destroyed and a corporal killed. Block 2 was evacuated at 15:00, and the garrison surrendered at 20:30.