You've entered an enormous building scoured by ocean tides and haunted by hulking machines, slowly rusting away. It looks like the set for a post-apocalyptic movie, but it's actually a real-life Soviet submarine base, left over from the Cold War.
Kraternyy, a secret base (1987-1994) on an uninhabited volcanic island named Simushir, Kuril Islands
(via Austronesian Expeditions)
Object 825, a hydraulic engineering structure for nuclear defense and a shelter for fourteen submarines in Balaklava, Ukraine
Bunker Alsou or Object 221 (also known as PCP BSF, Protected Command Point Black Sea Fleet) in the Crimea