pdk42
One of the "Eh?" team
Ah, now - that's a very interesting question...
It's actually the roof space in one of the giant submarine repair buildings built by the Nazis at Lorient in Brittany. It's an amazing structure. Along with two others, it was constructed in only eleven months between Feb 41 and Jan 42 and became a thorn in the flesh for Britain in its battle with the German U-boat fleet in the North Atlantic.
The shot you're looking at shows a void between two roof layers. The lower layer is a solid reinforced concrete slab about 1.5m thick. The upper layer consists of sequential concrete slabs separated by a metre or so. The void between them is about 3m high and has several apertures along its sides. The idea is that a direct bomb impact would destroy part of the upper roof but the main blast would be directed along the void and out of the apertures leaving the main roof intact. It was very successful and was never penetrated by any bomb of the time. Along with other defences it and its two siblings were never compromised despite relentless air attack by the British.
In 1943 Churchill decided that the only way to put the base out of action was to instead raze the town of Lorient to rubble and so deny the Nazis access to essential infrastructure and resources. Attacks in Jan and Feb 1943 destroyed more than 90% of the town. Whether it actually made any difference is debatable since the U-boat was eventually defeated by a combination of sonar, radar and the cracking of the German Enigma codes.
After the war the French adopted it as one of their submarine bases, pensioning it off only in the 1990s when larger bases were needed for the newer nuclear submarines. The local authorities spent several years figuring out what to do with it. Demolition and clearing of the site was ruled out for reasons of cost - there being over 1.5 million cubic metres of re-inforced concrete in the complex as a whole. Instead it's now home to an array of activities from tourism (such as guided tours like the one I took to take the photo) to light industry.
Long explanation - but you did ask...
