Fun fact: her sister, U-2540, once stayed submerged for 12 years before returning to German service.

Okay, technically she was scuttled in 1945 and refloated in 1957 but I like my version better.

Eboreg said in comment #2600439:

Fun fact: her sister, U-2540, once stayed submerged for 12 years before returning to German service.

So that's where that one joke comes from.

Incineration said in comment #2600464:

So that's where that one joke comes from.

What joke?

Eboreg said in comment #2600472:

What joke?

Short version is that a brit, an american and a german are arguing about their subs. The brit argues about how long theirs can stay submerged, then the american claims theirs can last longer, and just as the german thinks to himself that theirs can't stay underwater for that long a sub surfaces and out comes a nazi asking whether the war is already over.