I recently visited the Abri Sadi-Carnot, a tunnel running for more than half a kilometre from the arsenal at the port to the centre of Brest, the only city in France to construct massive capacity shelters during WWII. This one was built between 1941 and 1942 as a refuge against allied air raids for the Germans and civilian population alike. The lower end is accessed directly from the Boulevard de la Marine, but 154 internal steps were needed to reach the exit at the other extreme, the end assigned to the local population. Despite successfully saving many lives during bombing raids aimed at the submarine base, it suffered the historic irony in September 1944 of causing hundreds of deaths thanks to a horrendous accident - one waiting to happen considering the German practice of stocking munitions and petrol in their lower end of the shelter. The resulting fireball and asphixiating gases sped through the tunnel, leaving mounds of corpses on the staircases to witness a stampede for safety.
During the Cold War, the Abri Sadi Carnot was adapted to a nuclear shelter, the radiation-proof doors still in situ today.
Writer living in Finistere, blogging about Breton history and landscape. Published work includes many books and articles on Brittany's complex past, real and legendary, walking guides and fiction. Latest books: Spirit of Place in Finistere (landscape-writing, 2017, also in French edition) and a new collection of Finistere walks. I also teach Breton history, give talks about Brittany and guide tours all over the region. See my website wendymewes.com