As far as naval "architecture" (in terms of shore-structures rather than boats) goes, I think the WW2 submarine pens at Lorient got most everything beat.
This is how you build submarine pens when the ground is too tough for digging out a basin but you absolutely, positively need it there:
But if you like tunnels, check out this underground munitions factory:
Klick me for more pictures
Built in WW2, used as a munitions depot by the east-German forces from the 70s onwards, and after it was handed over to the west-German Luftwaffe it became the storehouse for
all of the superfluous east-German paper-currency, savings-books, fuel-vouchers etc. after 1991.
Unlike many other large underground structures which were backfilled or sealed, it's still there, some private owners are trying to figure out what to do with it...
This would have been part of an underground Autobahn intersection underneath <s>Berlin</s> "Germania".
Became an air raid shelter and was abandoned after the war.
They built a lot of crazy stuff, but like in the USSR, it was built on the blood and sweat of prisoners, which for me puts a big damper on admiring the engineering.
