Building of this U-boat pen began in early 1941.
At 300 metres long and 150 metres wide, 450,000 cubic metres of concrete were required in its construction.
It has 14 cells or chambers, eight of which were used as dry docks for repairing U-boats, while six larger docks allowed vessels to stay afloat.
Two U-boat flotillas were housed at the base. The complex stretches over an area of four hectares in total, and its upper slab of concrete is four metres thick.