A Russian cargo ship, which was likely carrying two nuclear reactors for submarines, possibly destined for North Korea, suffered a series of explosions and sank under unclear circumstances about 60 miles off the coast of Spain, according to a CNN investigation.
The fate of the Ursa Major remains shrouded in mystery after it sank on 23 December 2024. However, according to the CNN report, it may indicate a rare and high-risk intervention by a Western military force aimed at preventing the shipment of upgraded nuclear technology from Russia to its key ally, North Korea. The ship departed just two months after Kim Jong Un sent troops to assist Moscow’s invasion of Ukraine.
Intense recent military activity around the wreck has increased the mystery regarding its cargo and destination. US nuclear detection aircraft have flown twice over the now sunken ship in the past year, according to publicly available flight data. The wreck was also visited, one week after the sinking, by a suspicious Russian spy ship, which caused four additional explosions, according to a source familiar with the Spanish investigation into the incident.
The Spanish government has said very little, releasing only a statement on 23 February after pressure from opposition lawmakers. It confirmed that the ship’s Russian captain told Spanish investigators that the Ursa Major was carrying “components for two nuclear reactors, similar to those used in submarines,” and that he was not sure whether they contained nuclear fuel.
The sequence of events that led the Ursa Major to the bottom of the Mediterranean remains unclear. According to the Spanish investigation, as described by a source familiar with its contents, a rare type of torpedo may have been used to breach the ship’s hull. The incident occurred in the final weeks of Joe Biden’s presidency, when the war in Ukraine was in a phase favourable to Moscow and there was a strong US desire to avoid direct escalation with Russia.
The Ursa Major, also known as Sparta 3, had been used to evacuate Russian equipment from Syria and docked on 2 December at the fuel port of Ust-Luga in the Gulf of Finland before moving to a container facility at the docks of Saint Petersburg. The ship’s public cargo declaration stated that its destination was Vladivostok in the Russian Far East when it departed on 11 December, carrying two large “manhole covers,” 129 empty containers, and two large cranes.
In October of the same year, its owning company, state-linked Oboronlogistics, stated in a release that its ships had been authorised to transport nuclear material. Time-lapse video of the loading of the Ursa Major at Ust-Luga, analysed by CNN, shows containers being placed inside the hull, leaving a gap beneath where the “manhole covers” would later be installed.
The ship sailed along the French coast before aircraft and vessels of the Portuguese Navy monitored it in Portuguese waters, according to a Navy statement. Two Russian military ships, the Ivan Gren and the Aleksandr Otrakovsky, escorted the vessel, and on the morning of 22 December the Portuguese Navy ceased tracking it, the statement added.
About four hours later, in Spanish waters, the ship abruptly slowed, prompting Spanish rescuers to communicate via radio to check whether it was in distress, according to the Spanish government investigation conducted by maritime authorities in the southern port of Cartagena. The ship’s crew responded that everything was fine.
However, about 24 hours later, the ship suddenly deviated from its course and at 11:53 a.m. on 23 December issued an emergency distress call, according to the investigation. It had suffered three explosions on the starboard side, probably near the engine room, killing two crew members. The ship was left listing and immobile, as shown in video posted on social media.
The 14 surviving crew members evacuated the ship in a lifeboat and were later rescued by the Salvamar Draco, a Spanish rescue vessel. At 7:27 p.m., a Spanish military vessel arrived to assist. Half an hour later, however, one of the Russian military ships escorting the Ursa Major, the Ivan Gren, ordered nearby vessels to remain two nautical miles away and later demanded that the rescued crew be handed over immediately.
Spanish maritime rescue authorities insisted they had to carry out a rescue operation and sent a helicopter to the vessel to check for survivors. Footage seen by CNN shows a rescuer attempting to enter the ship’s engine room but finding it sealed. The Spanish rescuer checked living quarters for survivors and looked inside the ship’s containers, finding two filled with rubbish, fishing nets, and other equipment, according to the video.
The Ursa Major appeared stable and did not seem likely to sink immediately, according to the source familiar with the investigation. At 9:50 p.m., however, the Ivan Gren fired a series of red flares over the area, followed by four explosions. Four similar seismic signals were recorded at that exact time in the wider area, with a pattern resembling underwater mines or open quarry explosions, the Spanish National Seismological Network told CNN.
At 11:10 p.m., the Ursa Major was reported to have sunk, according to the source familiar with the Spanish investigation.
The 14 surviving Russian crew members were taken ashore in the port city of Cartagena, where they were questioned by Spanish police and investigators. The Russian captain was reluctant to speak about the ship’s alleged cargo, fearing for his safety, according to the Spanish government statement to opposition lawmakers.
The captain was pressed “to clarify what he meant by the term ‘manhole covers,’” the items listed in the ship’s original cargo declaration, the statement added. “He eventually admitted that they were components of two nuclear reactors, similar to those used in submarines. According to his testimony, and this cannot be confirmed, they did not contain nuclear fuel.”
The source familiar with the investigation said the Russian captain, named as Igor Anisimov, believed the ship was to be diverted to the North Korean port of Rason to deliver the two reactors. The Spanish investigation analyses the unlikely choice of a global sea voyage to deliver cargo consisting of two cranes, 100 empty containers, and two large manhole covers from one Russian port to another, despite the extensive rail network across the country. The investigation argues that the cranes were on board to assist with the delivery of sensitive cargo upon arrival in Rason.
The ship’s crew returned to Russia a few days later. CNN contacted a man with the same name and appearance as the Russian captain. He denied any involvement with the Ursa Major and said he is retired. Four days after the sinking, the ship’s owner, Oboronlogistics, described the incident as a “targeted terrorist attack” and said three explosions occurred. A 50-by-50-centimetre hole was found in the ship’s hull, with the damaged metal bent inward. “The ship’s deck was covered in fragments,” the company’s statement added.
A week later, according to the source familiar with the investigation, the Russian military returned to the site. The Yantar—officially a Russian research vessel but accused of espionage and sabotage in NATO waters—remained over the wreck of the Ursa Major for five days, the source said, before four more explosions were detected, possibly targeting the remains of the ship on the seabed.
Maritime tracking data from intelligence firm Kpler shows that the Yantar was in the area in January of last year, anchoring in Egypt and then Algeria, and on 15 January transmitted a position signal 20 kilometres from the last known location of the Ursa Major.
Critical evidence on the seabed of the Mediterranean
Some details of the Spanish investigation into the incident were first published by the local Cartagena newspaper La Verdad in December, prompting a series of questions from Spanish opposition lawmakers. MP Juan Antonio Rojas Manrique told CNN: “When someone does not provide the information you request clearly and fully, you at least suspect they are hiding something… of course.”
In its statement to lawmakers, the Spanish government said the wreckage of the Ursa Major lies at a depth of about 2,500 metres and that recovering the data recorder from such a depth “is not possible without significant technical resources and risks.” Experts have questioned why the government considers the operation so dangerous, given that no radioactive material is involved.
Rojas, a former merchant navy captain, also expressed scepticism, telling CNN: “Today, black boxes usually float to the surface with a locator so they can be found in any accident. I think someone has the black box. But we do not know whether it is Spain or whether the Russians themselves have located it.”
The US military has also shown interest in the area, sending twice an advanced aircraft for detecting nuclear traces, known as the WC135-R and based in Nebraska, over the incident site after the sinking of the Ursa Major—once on 28 August last year and again on 6 February this year, according to publicly available flight data.
A spokesperson for the 55th Wing at Offutt Air Force Base in Nebraska, Chris Pierce, confirmed that the aircraft’s role usually “supports the collection and analysis of nuclear debris.” “We cannot provide additional details regarding specific flight paths, mission findings, or any coordination with partners,” he added. Another WC135-R had flown a relatively similar route 13 months before the sinking of the Ursa Major, suggesting that interest in the area may have predated the sinking or been routine.
It is not clear whether these two rare and costly flights—using aircraft that usually operate secretly and are used to detect nuclear activity in the Russian Arctic or around Iran—detected any trace of contamination from the Ursa Major wreck. The Spanish government has given no indication that it fears radiation along the country’s southern coast, a popular tourist destination, and no evidence to that effect has emerged.
Possible exchange of nuclear secrets
The claim that North Korea was the likely recipient of the two reactors allegedly on board the ship comes after the isolated regime of Kim Jong Un released images in December 2025 of its first nuclear-powered submarine. The static images, showing Kim smiling, only display the sealed hull of the vessel and provide no evidence of a functioning nuclear reactor inside.
Mike Plunkett, senior naval platforms analyst at Janes, a defence intelligence company, said it was unlikely that the reactors, if new, would have been transported with fuel inside them. “If these reactors come from decommissioned submarines, then they will be radioactive, although obviously not to the same extent as if they were fully loaded with fuel,” he said.
Any Russian decision to transfer this technology to North Korea “is not taken lightly and is something done only between very close allies,” he added. If true, “this is a major move by Moscow.” He described such a development as “potentially very worrying, particularly if you are South Korea.”
The Spanish investigation, as described to CNN, identifies North Korea as a strategic ally of Russia, and notes that Pyongyang has openly called on Moscow to share its technical expertise in the nuclear field. It is possible that such requests increased after at least 10,000 North Korean troops travelled to Russia in October 2024 to fight the Ukrainian incursion in the Kursk region.
The investigation states it is possible that the reactors being transported were of the VM-4SG model, commonly found in Russian nuclear-powered ballistic missile submarines of the Delta IV class, but provides limited evidence to support this claim.
CNN obtained satellite images from Vantor of the Ursa Major docked at the eastern end of the Ust-Luga port in the Gulf of Finland on 4 December 2024. Geolocated time-lapse videos posted on the ship owner Oboronlogistics’ account show the vessel being loaded there with containers and cranes.
After the sinking, the Russian newspaper Kommersant reported that the Ursa Major was carrying port cranes and hatches designed to cover nuclear reactors for a new icebreaker being built in Vladivostok. The report did not mention the two white objects.
What created a hole the size of a pillow in the hull of the Ursa Major
The Spanish investigation also examines the initial strike that caused the Ursa Major to deviate from its course and list, according to the source familiar with the report. The Russian captain told investigators that he did not hear any impact or explosion on 22 December, when the ship suddenly slowed. Only 24 hours later were there three explosions near the engine room, killing two crew members—the second engineer Nikitin and engineer Yakovlev—whose bodies were not found.
Plunkett, the Janes analyst, suggested that a magnetic mine attached to the hull was a more likely explanation for the size and position of the hole. “It sounds like an explosive shaped-charge device placed on the hull by someone or something,” he said.
The ship’s Russian owners, Oboronlogistics, as well as Russian, Spanish, and British military authorities did not respond to requests for comment. The Pentagon declined to comment. Many Western security and intelligence officials contacted by CNN described the incident as strange or suggested that some conclusions of the Spanish investigation were exaggerated, but did not provide an alternative, benign explanation for the initial explosions that struck the ship or for the strong Russian response to the situation.
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