The Gros Ouvrage of Monte Grosso lies at an imposing 967 metres and was constructed between 1931 and 1935. It cost around 40 million france, of which over a tenth was for the access road alone. It is unusually large for forts on the Alpine section of the Maginot Line and resembles more those of the northern sector. It was positioned to protect the northern approaches to Nice.
In addition to the entrance block it held four artillery blocks and two infantry/observation blocks. The two main turrets were retractable, unusually featuring a howitzer of 135mm. The fort saw action against Italian positions in June 1940 and also received incoming fire although it remained operational.
When visited in 2002 it was obvious that much of the entrance level had been stripped for scrap metal. There was evidence of burning, presumably to remove outer insulation from copper wiring. At this time the upper galleries were in better condition.