Ouvrage Fermont, Military museum in Montigny-sur-Chiers, France.

Ouvrage Fermont is an underground military fortification in Montigny-sur-Chiers with multiple levels and corridors connecting different combat positions across its grounds. The structure includes reinforced blocks housing gun emplacements, control centers, and barracks for the personnel who operated it.

The fortification was built in the early 1930s as part of the Maginot Line, a French defensive system along the German border. It served as a fortified position during World War II until the military situation changed dramatically for French forces.

The fort's name comes from the Fermont farm on whose land it was built. Its 553 soldiers and 19 officers of the 149th Fortress Infantry Regiment and 152nd Position Artillery Regiment lived underground, with barracks, kitchens, infirmary and latrines near the junction of the two entry galleries. A 60 cm narrow-gauge railway carried munitions from the entry to the combat blocks.

Visitors travel through the underground sections on a narrow-gauge railway and can explore different rooms and functional areas. Most corridors are straightforward to walk through, though stairs and tight passages lead to certain sections.

The concrete wall bearing the marking "A2 FERMONT" is one of the two entrance blocks, set a kilometre south of the combat blocks. Both entrances are built as vertical shafts, since the local terrain ruled out an inclined gallery, and each required an elevator to reach the 30-metre-deep gallery. Each is guarded by a JM/AC 47 embrasure and two GFM cloches.