A Russian cargo ship that suffered a series of mysterious explosions before sinking approximately 60 nautical miles off Spain's southeastern coast may have been carrying nuclear submarine reactor components destined for North Korea, according to a CNN investigation and Spanish government documents. The incident, which remains shrouded in secrecy, may mark a rare and high-stakes intervention by a Western military to prevent Russia from transferring sensitive nuclear technology to a key ally.
The Ursa Major (also known as Sparta 3), a 142-meter-long, Russian-flagged cargo ship owned by the state-linked Oboronlogistics company, departed St. Petersburg on December 11, 2024, ostensibly bound for Vladivostok in Russia's Far East. Its public manifest listed "non-dangerous merchandise": 129 empty shipping containers, two large Liebherr cranes, and two oversized "manhole covers." However, satellite imagery and loading footage analyzed by CNN showed containers being placed inside the hull with a deliberate gap left below where the large white objects—later identified as reactor components—would sit.
In October 2024, Oboronlogistics publicly stated its vessels were licensed to carry nuclear material. The ship, a veteran of Russia's military campaign in Syria, where it evacuated Russian equipment, had docked at Ust-Luga in the Gulf of Finland before moving to St. Petersburg's container facility for final loading.
The vessel traveled down the European coast, escorted by two Russian military ships, the Ivan Gren and Aleksandr Otrakovsky. Portuguese naval assets tracked it through their waters before dropping the escort on the morning of December 22. Approximately four hours later, in Spanish waters, the Ursa Major slowed dramatically. Spanish maritime authorities radioed to check on the vessel; the crew replied that all was well.
But about 24 hours later, at 11:53 a.m. UTC on December 23, the ship issued an urgent distress call. The crew reported three explosions on the starboard side, likely near the engine room, which killed two crew members (identified as Second Mechanic Nikitin and Mechanic Yakovlev) and left the vessel listing and immobile. Social media footage showed the ship in distress as the 14 surviving crew members abandoned ship in a lifeboat.
Spain's maritime rescue service, Sasemar, dispatched a helicopter, fast rescue boat, and tugboat. The survivors were picked up by the Salvamar Draco. At 7:27 p.m., a Spanish military vessel arrived to assist. However, at approximately 8:00 p.m., the Russian warship Ivan Gren—one of the escorts—ordered all nearby vessels to withdraw to a distance of two nautical miles and demanded the immediate return of the rescued crew. Spanish authorities insisted on continuing rescue operations and sent a helicopter to check for additional survivors. Footage seen by CNN shows a rescuer attempting to enter the sealed engine room and peering into containers that held only trash and fishing nets.
According to a Spanish government document released in response to parliamentary questions, Ivan Gren then launched a series of red flares over the Ursa Major at 9:50 p.m. A report in the Murcia newspaper La Verdad suggested the flares could have been deployed to blind the infrared channels of intelligence satellites monitoring the incident.
Moments later, four additional explosions occurred. The Spanish National Seismic Network registered four similar seismic signatures at that exact time and location, with patterns resembling underwater mines or overground quarry blasts. By 11:10–11:20 p.m., the Ursa Major had sunk. It now rests at a depth of approximately 2,500 meters (8,202 feet). Two crew members are confirmed dead; 14 were rescued.
Under questioning by Spanish investigators, the Ursa Major's captain, Igor Anisimov, eventually conceded that the "manhole covers" listed on the manifest were in fact "components for two nuclear reactors similar to those used by submarines." He stated he was unsure whether they contained nuclear fuel. The Spanish government's statement to opposition lawmakers confirmed this testimony but noted it could not be independently verified.
Investigators had previously noticed two massive blue containers—each estimated to weigh about 65 tonnes—on the ship's stern in satellite photographs. A report in La Verdad noted that transporting such heavy, oversized cargo along the winding roads of Russia, Kazakhstan, Uzbekistan, and Afghanistan would be nearly impossible, "justifying a voyage of more than 15,000km by sea."
A source familiar with the Spanish investigation told CNN that Captain Anisimov believed he would be diverted to the North Korean port of Rason to deliver the two reactors. The Spanish investigation analyzed the unusual choice of a round-the-world sea route to deliver cargo between two Russian ports despite the country's extensive rail network, suggesting the cranes aboard were intended to assist in offloading sensitive cargo upon arrival in Rason.
A critical piece of evidence is a 50cm by 50cm (20-inch by 20-inch) hole found in the vessel's hull, with damaged metal facing inward. Oboronlogistics, in a statement calling the incident a "targeted terrorist attack," noted the deck was "strewn with shrapnel."
The Spanish investigation, as described to CNN, proposes the hole was likely made by a Barracuda-type supercavitating torpedo. This rare weapon, believed to be possessed only by the United States, a few NATO allies, Russia, and Iran, fires air ahead of the projectile to reduce water drag, enabling extremely high speeds. Some models can pierce a hull without an explosive charge, potentially creating a noiseless impact. The source familiar with the investigation said this theory would align with the sudden, unexplained slowing of the Ursa Major on December 22—24 hours before the crew-reported explosions—and the size and inward-facing damage of the hull breach.
However, experts consulted by CNN offered alternative views. Mike Plunkett, senior naval platforms analyst for Janes, suggested a limpet mine with a shaped charge was a more plausible explanation for the hole's size and location, stating it "sounds like a shaped-charge explosive that was placed against the hull by somebody or something."
The mystery deepened in the weeks and months following the sinking. According to a source familiar with the Spanish investigation, the Russian research vessel Yantar—accused of espionage and disruption in NATO waters—visited the wreckage site approximately one week after the sinking and remained for five days. During this period, four additional explosions were detected, possibly targeting the remains on the seabed. Maritime tracking data from Kpler shows the Yantar was in the region in January 2025, mooring in Egypt and Algeria before sending a position ping just 20 km from the Ursa Major's last known location.
Simultaneously, the U.S. military demonstrated interest in the site. Public flight data shows a rare WC-135R "nuke sniffer" aircraft, based in Nebraska and designed to detect nuclear debris, overflew the area twice: once on August 28, 2025, and again on February 6, 2026. A spokesman for the 55th Wing confirmed the aircraft's role "supports nuclear debris collection and analysis" but declined to provide mission-specific details. Another similar flight occurred 13 months before the sinking, suggesting a possible pre-existing interest in the area.
The incident occurred against a backdrop of a deepening Russia-North Korea alliance. The Ursa Major set sail just two months after North Korean leader Kim Jong-un sent an estimated 10,000 troops to assist Russia's invasion of Ukraine. In December 2025, North Korea released images of its first claimed nuclear submarine, though the pictures showed only a sealed hull with no evidence of a functioning reactor.
Mike Plunkett of Janes noted that if Russia transferred reactor technology to North Korea, "it's a major move by Moscow" and "very troubling, potentially, particularly if you're South Korea." He added that new reactors would likely not be shipped with fuel, but decommissioned units would still be radioactive. The Spanish investigation suggests the reactors transported were likely the VM-4SG model, commonly used in Russia's Delta IV-class ballistic missile submarines, though it provides limited evidence for this specific claim.
Russian state media outlet Kommersant offered a different explanation post-sinking, reporting the Ursa Major was carrying port cranes and hatches for a new icebreaker being built in Vladivostok—a report that did not mention the two large white objects central to the nuclear cargo theory.
The Spanish government has been reticent, releasing minimal information only after pressure from opposition lawmakers. It stated that recovering the ship's data recorder from 2,500 meters depth "is not possible without significant technical resources and risks." Experts have questioned this rationale if no radioactive material is involved. Lawmaker Juan Antonio Rojas Manrique, a former merchant marine captain, expressed skepticism: "Nowadays black boxes usually float to the surface with a locator… I think someone has the black box. But we don't know whether it's Spain or if the Russians themselves have located it."
Spain's interior, foreign, and defense ministries, along with the Russian, British, and U.S. militaries and Oboronlogistics, did not respond to requests for comment. The Pentagon declined to comment. Multiple Western security and intelligence officials described the incident as "strange" or suggested some of the Spanish investigation's conclusions were "far-fetched," but none provided an alternative, benign explanation for the initial blasts or the acute Russian reaction.
The 14 surviving crew members were debriefed by Spanish police in Cartagena before being returned to Russia days later. CNN contacted a man bearing the name and likeness of Captain Igor Anisimov; he denied involvement with the Ursa Major and stated he was retired.
The sinking of the Ursa Major presents a complex puzzle intertwining maritime disaster, covert military action, and high-stakes nuclear proliferation. Whether the hull was breached by a supercavitating torpedo, a limpet mine, or another device, whether the cargo was indeed submarine reactor components bound for North Korea, and whether a Western power intervened to stop the transfer—these questions remain officially unanswered.