At the top of Janus Mountain built between 1886 and 1903 is an underground network added as part of the Maginot Line.
The mountain top has been successively outfitted over the years to counter military threats. The renovations were stopped in 1935 according to the Franco-Italian alliance but resumed in 1938 when Italy moved closer to Germany. The fort was attacked in 1940. Today, it is in ruins but frequented by hikers and skiers, as it is very close to the Montgenèvre resort.