Hacker News Reader

Island of Vis in Croatia has one, legacy of Yugoslavia, you can swim in. It's a fun experience.

There's a quite big one in Sweden also, built in the 50's that is still active. https://en.wikipedia.org/wiki/Musk%C3%B6_naval_base

Where exactly?

43.07809 16.18140

Got curious and went looking, this looks like the one:

Ex. Yugoslavian military shelter for submarines D117, 21480, Rogačić, Croatia

That's the one. Island is relatively small and has interesting military installations on it.

Correct! I have been there.