At one point during the tragic episode of the sinking of Kursk, a norwegian tabloid newspaper (Dagbladet) reported that the submaring held 6 civilian scientists in addition to the 118 military crewmembers. The story was confirmed by another norwgian newspaper (Aftenposten), this time not a tabloid. This piece of information was never mentioned again by any media, for confirmation or otherwise. However, in Russia, rumors abound that there was six civilian scientists present that were performing experiments of an unknown nature.
Other rumors had it that the captain of the Kursk was violently drunk during the whole of the excercise. A notion that, according to russian civilians, holds true for most of the russian higher officers.
The idea that the Kursk had collided with an american sub; the Missisippi, seemed to be popular for a while. It was particularly supported by a sattelite photography of a norwegian naval base where the american submarine could be made out, dokking for what was assumed to be repairs. American officials claimed the photograph was old and had nothing to do with the present excercice. Unfortunatly, the picture also depicted several items that had been built only shortly before the Kursk-incident, disproving the 'western explanation'. This story was also discontinued by norwegian media.
A relativly unpopular theory incriminated another russian ship, name unknown. According to a variety of norwegian newspapers, there was a surface ship involved in the exercice, just above where the Kursk was sunk. This ship was supposed to launch anti-sub weaponry agains Kursk. These weapons were not supposed to hit or explode. Regardless, a norwegian listening-station (a civilian station listening for seismic activity, a hopeless job as Norway doesn't have any) picked up one faint explosion followed by a very large explosion around the time of the sinking. The surface ship could not account for all it's weapons during debriefing.
Still, the most popular theory, and the one that'll go down in history as the Truth, is the one about the exploding torpedoes. Which seems the most likely. Which has the least implications. Let's leave it at that.