A Russian cargo ship, believed to have been carrying two nuclear reactors for submarines, has sunk in mysterious circumstances off the coast of Spain, and the case is now drawing serious attention from Western militaries, intelligence circles and maritime investigators. The vessel, the Ursa Major, went down about 60 miles off Cartagena after a string of explosions, in what one investigation suggests may have been a covert attempt to stop sensitive nuclear technology reaching North Korea.
The ship’s fate has remained largely hidden since it sank on 23 December 2024, but the details now emerging point to an incident that is far bigger than a routine shipping disaster. According to reporting and investigation findings reviewed by SA Report, the vessel may have been carrying cargo tied to Russia’s military and nuclear programme at a time when Moscow’s ties with Pyongyang were deepening. That alone makes the incident geopolitically explosive.
What makes this story even more unusual is the trail of military activity that followed the sinking. Public flight data shows that a US WC135-R “nuke sniffer” aircraft has flown over the wreck site twice in the past year. Those aircraft are typically used to detect radioactive particles in the air and are normally associated with the monitoring of nuclear debris after a suspected incident. Their appearance over the wreck area has only intensified speculation about what exactly was on board.
There is also the matter of a suspected Russian spy ship, which reportedly visited the remains of the Ursa Major a week after the sinking and triggered four further explosions, according to a source familiar with the Spanish investigation. That detail, if confirmed, would suggest the site itself remained of strategic interest long after the crew had been rescued and the ship had disappeared beneath the Mediterranean.
Spanish authorities have said very little publicly, but their limited statement has been enough to fuel more questions than answers. On 23 February, after pressure from opposition lawmakers, the Spanish government confirmed that the Russian captain had told investigators the ship was carrying “components for two nuclear reactors similar to those used in submarines”. He reportedly said he was not sure whether they were loaded with nuclear fuel.
The Ursa Major, also known as Sparta 3, was no ordinary cargo vessel. It had previously been used in Russia’s military operations in Syria, where it helped evacuate Russian equipment. Before its final voyage, the ship docked at Ust-Luga in the Gulf of Finland on 2 December, then moved on to a container facility in St Petersburg. Its manifest showed it was supposedly heading to Vladivostok with two large manhole covers, 129 empty shipping containers and two large Liebherr cranes.
That cargo has raised eyebrows. The ship’s owner, Oboronlogistics, a state-linked company, had previously said its vessels were licensed to carry nuclear material. Video footage analysed by CNN appears to show containers being loaded into the hull, with space left for the items described as manhole covers. Investigators are now asking why a cargo of that nature would travel the long way around Europe when Russia has an extensive rail network connecting its ports and industrial centres.
The route itself was heavily monitored. The ship moved along the French coast and then through Portuguese waters, where it was tracked by Portuguese Navy aircraft and vessels, according to official statements. It was escorted by two Russian military ships, the Ivan Gren and the Aleksandr Otrakovsky, until the Portuguese navy lost visual contact on the morning of 22 December. A few hours later, things began to unravel.
Ursa Major sinking investigation raises nuclear cargo fears
Around four hours after leaving Portuguese surveillance, the Ursa Major slowed sharply while in Spanish waters, prompting rescuers to radio the vessel and ask if it was in distress. Its crew insisted everything was fine. But by the next day, at 11:53 a.m. UTC on 23 December, the ship issued an urgent distress call after suffering three explosions on its starboard side, likely near the engine room. Two crew members were killed, and the ship was left listing and unable to move.
The 14 surviving crew members abandoned ship in a lifeboat and were later picked up by the Salvamar Draco, a Spanish rescue vessel. A Spanish military ship arrived not long after, but the situation became even more tense when the Russian escort vessel Ivan Gren ordered nearby ships to keep two nautical miles away and demanded the return of the rescued crew.
Spanish rescue authorities refused to be pushed aside, insisting on carrying out their own operation. A helicopter was sent to scan the wreck for survivors. Video seen by CNN shows a rescuer trying to enter the engine room, only to find it sealed. The same footage reportedly shows the crew checking the living quarters and looking inside containers, where they found items including trash, fishing nets and other equipment.
Then came the most troubling development. At 9:50 p.m., the Ivan Gren fired red flares over the scene, and four explosions followed. The Spanish National Seismic Network registered four matching seismic signatures at the same time and in the same area, a pattern experts said resembled either underwater mines or quarry blasts. By 11:10 p.m., the ship was reported sunk.
The Spanish investigation, as described to reporters, suggests the initial damage may have come from a rare Barracuda supercavitating torpedo or possibly a limpet mine attached to the hull. A 50cm by 50cm hole was found in the vessel’s side, with metal bent inward and shrapnel scattered across the deck. That damage pattern has led some investigators to believe the ship may have been deliberately targeted.
The captain, identified as Igor Anisimov, reportedly believed the ship had been diverted to Rason, a North Korean port, to deliver the reactors. The investigation further suggests the odd cargo of cranes and container space may have been intended to support the transfer of sensitive equipment on arrival. It also notes that the supposed reactors were likely VM-4SG models, commonly used in Russia’s Delta IV-class nuclear-powered submarines.
There are still major gaps in the public record, and some experts have questioned the Spanish government’s reluctance to release more detail. If there is no radioactive material involved, critics argue, why is recovering the vessel’s data recorder from a depth of about 2,500 metres being treated as too risky? A former merchant marine captain, Juan Antonio Rojas, has also cast doubt on the official silence, saying he suspects someone has the black box.
For now, the wreck remains on the seabed, and the full truth of what happened is still buried with it. What is clear is that the Ursa Major sinking investigation has turned into a case that links ship explosions, suspected espionage, nuclear technology and the shadowy relationship between Russia and North Korea. Until the evidence is fully released, the sea floor off Spain may continue to hold the answers.