Ship mysteries are a dime a dozen. Many ships have vanished over the years, without a trace. Like the entire Proteus class, outside of Langley. Similarly, many ships have had odd sinkings. Maine is the one most people point to, though that's pretty well solved at this point. Mutsu is another option, in this regard.

And then there's the exGiulio Cesare. Renamed to Novorossiysk after transfer to the Soviet Union, this ship had a troubled life. Constant refits and visits to the drydock. A maintenance nightmare and an already obsolete ship.

Things can, and generally do, always get worse in the Soviet (or Russian) Navy.

Novorossiysk would succumb to a massive explosion in 1955. The source of this explosion has been argued ever since. The most probable answer, and the generally accepted one, is an old German mine.

However, conspiracy theories abound. Generally featuring Italian frogmen acting like they're in a James Bond film. Those...range from unlikely, to plain silly.

This video will cover them all, regardless.

Axis War Prize Video:    • Under The Red Flag – Axis Ships in So...  

USNI Article: https://www.usni.org/magazines/naval...

Further Reading:
https://www.amazon.com/RussianSoviet...
https://www.amazon.com/WarshipsSovie...
https://www.amazon.com/ItalianBattle...
https://www.amazon.com/SpoilsWarEne...