Funny, I was reading "A Measureless Peril" this morning. It's a book about the Battle of the Atlantic. In the chapter "Doenitz goes to France" the author remarks that the only remaining vestiges of the Thousand Year Reich that may really last a thousand years are the sub pens the Nazis built in Lorient, Brest, St.-Nazaire and Bordeaux. They consumed a million tons of steel and 14 million cubic feet of concrete in constructing the sub bases. (There are 4 million tons of concrete in Hoover Dam by comparison).

Despite constant bombing,

"None of the bases suffered any serious harm. When in 1987, filmakers needed a set for the popular submarine movie Das Boot, they simply moved into La Pallice, where the old machinery stood intact and dim Gothic lettering still proclaimed it exhortations and prohibitions from the unmarred walls"

The Battle of the Atlantic was the longest continuous battle of WW2.