Plan Caval is located amongst a number of abandoned and ruined 19th century barrack blocks. These had nothing to do with the Maginot Line, but belong to an earlier generation of fortifications on the Authion Plateau.
The plateau formed a kind of bastion between the Tende region and the Mediterranean (Sospel-Menton). It acted as a natural fort and was used for the defence of Nice. The plateau can be easily reached and an enemy who occupies Authion can reach Nice via the Col du Turini.
In 1793 it was the site of violent battles when the French General Brunet unsuccessfully tried to break through the defence of the Sardes. After that the French reinforced the plateau and after 1870, when General Séré de Rivières came up with his ideas for an national defence of the French territory, things were accelerated.
The creation of the Alpine Army was the starting point for the construction of defensive works in the high mountains. Several barracks were constructed at Peira-Cava, Cabanes-Veilles, Tete de l’Authion and Plan Caval. Today Authion is a national park.
Plan Caval is an unfinished gros ouvrage with 6 blocks planned but only three built. These blocks were never finished and the fort was never armed. The fort can be entered through the emergency escape hatch in Block 4 located at the bottom of the fosse, there was a large pile of stones making it easy to climb down. The small block is partially camouflaged as it’s clad in stone. It would have been only lightly armed with no heavy guns or mortars. A ladder alongside the lift shaft gives access to the corridor 20 feet below. Although the lift shaft with its gate has been constructed the lift and its associated machinery have never been installed The passage is concrete lined and very clean. It is necessary to watch the floor carefully as there are a number of uncovered drains. After a short distance there is a crossroads, to the left is a collapsed or backfilled shaft to the surface and to the right a short passage leads to Blocks 5 & 6.
There is a long tunnel straight ahead with a dog leg and a defensive embrasure in a room to one side. The passage continues for about 75 yards eventually opening out into a series of unlined tunnels running left and right. A number of these have wooden pit props and one of them is very heavily propped with vertical supports and cross timbers. A number of passages have collapsed but it’s possible to climb over these collapses to reach a series of very high parallel tunnels. From here there was a tunnel to a blocked door, presumably the original planned entrance to Plan Caval. This was later located on the surface on a valley floor below the road.