[jrbancel]

Bordeaux faced a similar problem with a submarine base. It is much bigger though: 450,000 sq ft. It is now a cultural center. According to my dad, who was involved in the conversion, working with 22 feet thick walls was very challenging.

https://en.wikipedia.org/wiki/BETASOM

[batou]

There's a bunker in the UK [1] that I went to. Before they could allow visitors they had to add a fire escape. Apparently it took a couple of days to cut a hole in the wall for this due to how strong it is and that was state of the art diamond cutting equipment.

Good day out though.

[1] http://www.secretnuclearbunker.com/

[cafard]

The USAAF had its best shot with 500 lb bombs, which must have chipped a certain amount of concrete. Apparently it was not a good idea for a GI to walk into French port towns with a bomb group patch on his arm in 1944 and 1945.

[noinsight]

I had no idea the Italians had submarines let alone in a base in France. Makes sense though, obviously.