from sea to shining sea.

At this point, I haven’t seen any experts thinking of the GUR as a cause, they just reported about it since they keenly monitor everything Russian.

People write that it was transporting cranes. On the deck, I see large structures which match the description. We can’t see into the cargo hold, though.

GUR’s report was about “Sparta” developing fuel pump problems. Since “Ursa Major” has the alias “Sparta 3”, I think Ukraininans may have mixed up numbers in their report and it could have been “Ursa Major” which developed fuel pump problems.

If the above premise holds true (no guarantee)… next I would guess that during repairs, an accident ignited a detonating mix of fuel vapours and the engine room became a potato cannon. Or maybe a fuel tank in the engine room blew apart. RIP for anyone in that compartment, probably a crack in the hull and maybe some loose cargo as a bonus.

A more sinister scenario would be “cranes on the deck and ammo in the hold”. But ammo is a gift the typically keeps on giving - if the first items blows up, subsequent items enter a chain reaction. This was not the case.

A far-fetched scenario would be an Ukrainian bomb planted in St. Petersburg or Kaliningrad (the ship was heading to Syria according to TWZ. So I doubt it. If Ukrainians could plant bombs anywhere in Russia, they would probably prefer to blow up something more interesting (like maybe an air defense system being returned from Syria by plane, or maybe the Kerch bridge, or some railway connecting Russia to North Korea, or something to that tune - since that would get them immediate benefits).