This story is about plans to open a visitors' center with exhibitions in 2015 at what was then the largest building project in the world, absolutely bomb proof submarine pens for Germany's Type XXI boats. Though great resources were thrown into the project, including the lives of about 1,600 slave laborers, it never actually launched even a single submarine. The plan was for the vessels to be constructed 8k upriver on the Weser in sections and then barged down to the giant pens, codenamed Valentin, where they would be welded together and equipped. The British bombed the facility when it was about 90% complete in March 1945 with bunker buster bombs, causing two massive rips in the roof and destroying its usefulness. The Allies had waited for the program to near completion before bombing it.
I didn't see where anyone had posted a link yet to the story so I thought I would throw this out there.
http://www.welt.de/kultur/history/article13358686/Valentin-Der-Bunker-fuer-Hitlers-Super-U-Boote.html