The former Northern Fleet commander Vyacheslav Popov offered a theory (and there are many!) that a NATO submarine caused the loss of the Kursk where a 118 sailors lost their lives more than two decades ago.
https://www.rt.com/russia/540944-sinking-kursk-submarine-nato/
The catastrophic sinking of the Russian nuclear-powered Kursk submarine more than two decades ago was the result of a collision with a stricken NATO vessel in the Barents Sea, a former high-ranking navy chief has insisted.
The ‘Kursk’ sank on August 12, 2000 at a depth of 108 meters, claiming the lives of all 118 crew members and sparking the first major international crisis of Vladimir Putin’s presidency. An official investigation commissioned by the Russian government ruled two years later that the incident was the result of a torpedo explosion, which then triggered the detonation of ammunition on board.
However, in an interview with RIA Novosti, aired on Monday, former Northern Fleet commander Vyacheslav Popov offered a theory on how the incident might have happened. According to him, a vessel operated by a NATO power got too close to Moscow’s vessel, colliding with its bow and damaging the torpedo tube, which was followed by an explosion. The compartment then flooded, sending the sub to the depths of the sea.
“The submarine that collided with ‘Kursk’ was following it, apparently, but failed to ensure safety in the sea’s environment and all other conditions, approached too close, or the Kursk maneuver led to a loss of contact,” he said.
Popov claimed he knew the name of the sub belonging to the US-military led bloc “with a 90% probability.” However, he admitted he did not have sufficient available evidence to make the case publicly at present.
Some years ago I watched a French made documentary narrated by Peter Coyote on the subject. The documentary pointed out that the suspect circumstances surrounding the sinking of the Kursk. Putin was just elected as President and Russia and the it's Western "partners" had good relations at the time, and a cover up of the real circumstances may well have been on the cards. Who knows?
One of the things that popped up in the documentary was a large hole on the starboard side of the Kursk. The hole represents that of a torpedo that penetrated the hull from the outside. Cameras were quickly barred from taking any footage of the hole.
There were many "conspiracy theories" floated on the internet at the time but this one made quite a bit of sense to me.