Komsomolets
In the 1980s, the Soviet Union built a super submarine that was nothing like anyone has ever seen before. Fast and capable of incredible depths for a combative submersible, the submarine Komsomolets was introduced in 1984. Just five years later, Komsomolets and its nuclear weapons were at the bottom of the ocean, two-thirds of its crew killed in what was considered yet another example of Soviet incompetence. The submarine sank in 5,250 feet of water, complete with its nuclear reactor and two nuclear-armed Shkval torpedoes. From 1989 to 1998, seven expeditions were carried out to secure the reactor against radioactive release and seal the torpedo tubes. Russian sources claim that during these visits, evidence of “unauthorized visits to the sunken submarine by foreign agents” were discovered.
Pavlovsk Bay
This secret underground base was discovered by explorers. It is now disused, but it was originally dug to store many of the Soviet Union submarines. The base is believed to be able to withstand a nuclear attack, but it seems like it was abandoned before it was ever fully used due to the Cold War ending. The explorers who found it managed to gain access to the series of tunnels thanks to the frozen water brought about due to the Siberian weather.