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On top of the Janus Mountain built between 1886 and 1903 is an underground network added as part of the Ligne Maginot.
The mountaintop was successively equipped over the years to fight against military threats. Renovations were stopped in 1935 according to the French-Italian alliance but resumed in 1938 when Italy went closer to Germany. The fort was attacked in 1940. Today, it is in ruins but visited by hikers and skiers, as it is very close to the Montgenèvre resort.