The Bréhain works in the town of Bréhain-la-Ville, Meurthe-et-Moselle, are completely underground. They consist of 10 blocks branching off a roughly 4,600-foot underground gallery. These hydraulic turrets are the only parts of the fortification visible above ground.

This bunker in the town of Seltz is one of seven bunkers lining the Seltzbach River.

The Valette Pass barracks in the town of Roure, in the French Alps.

The entry to the Falkenberg shelter in the town of Philippsbourg, Moselle sat at the base of the Vosges Mountains.

The Hackenberg fortifications in Veckring, Moselle, include 19 combat blocks connected by underground galleries. An anti-tank trench is the main visible structure.

In Modane, Savoie, rock-hewn underground galleries connect an armored combat block to others.

These ruined barracks and entry blocks were part of the Restefond Pass fortifications in the French Alps, the highest part of the Maginot Line’s defense system.

An infrared aerial photograph from 1974 showing the Milles Fourches Fort and La Forca Fort, part of the Maginot Line in the French Alps. The image was projected on a wall and photographed in detail.

The Gebirgswald shelter sits in the town of Indweller, Bas-Rhin. It has seven entryways and a network of underground galleries. It was never completed.

At the front of the Maginot Line sat fortified houses that looked ordinary but were occupied by soldiers. This one in Pouru aux Bois,Ardennes was restored during the making of a period movie.

Remains of Maginot Line barracks in the Etroite Valley in the French Alps.

A foggy view of Mille Fourches Forg, photographed from La Forca Fort in the French Alps. The infantry works sat on two summits roughly 6,500 feet above sea level and defended the Royal Valley from an Italian invasion.