The Russian cargo ship Ursa Major sank in the Mediterranean Sea on December 23–24, 2024, after explosions, with reports suggesting it was secretly carrying VM-4SG naval nuclear reactor components destined for North Korea.
Spanish newspaper La Verdad has reported that investigators suspect the cargo was bound for North Korea to support its naval modernization efforts.
The vessel sank to a depth of roughly 2,500 meters between Spain and Algeria, sparking concerns of illegal nuclear proliferation.
The ship, part of a Russian “shadow fleet,” experienced engine room explosions and sank on December 23–24, 2024, in the Western Mediterranean.
Ursa Major was known as part of Russia’s so-called “shadow fleet”—vessels believed to be involved in arms and oil smuggling, often in violation of international sanctions.
The United States, South Korea, and several other governments have long suspected Pyongyang of receiving Russian technical expertise in return for munitions and, reportedly, troops for President Vladimir Putin’s war in Ukraine.
Spanish maritime authorities responded to Ursa Major’s distress call on December 23, 2024, finding the ship listing about 70 miles south of Cartagena, Spain.
The ship was later determined to have suffered three explosions in the engine room. Oboronlogistika, a Russia-controlled company that operated the ship, attributed this to a “terrorist attack.”
Rescuers evacuated 14 crew members; two others, who had been in the engineering compartment, were missing. The ship sank soon after.
Suspicion arose when the ship’s captain told investigators that the cargo consisted of more than 100 empty containers, parts for a Russian icebreaker, and a pair of massive cranes visible on deck. Also observed by spotting planes were two large, blue-tarped items on the deck, each estimated to weigh around 70 tons.
When pressed by investigators, the captain claimed these mystery items were simply “manhole covers.” However, according to reports reviewed by La Verdad, Spanish investigators later said they believed the objects were casings for VM-4SG nuclear submarine reactors, a Soviet-era design still in use aboard half a dozen Russian Delta IV-class ballistic missile submarines.
These reactors feature three-foot-thick steel lids designed to shield crews from ionizing radiation, which would account for their exceptional weight, according to The Maritime Executive. It remains unknown whether any radioactive material or nuclear fuel was aboard, but this was deemed unlikely.
The report notes that Ursa Major was purportedly bound from St. Petersburg to Russia’s eastern port of Vladivostok—a highly circuitous route if only to deliver icebreaker parts and empty containers.
Spanish investigations concluded the ship was transporting two nuclear reactor housings intended for North Korea’s submarine program.
While Russia cited mechanical failure, investigators observed hull damage consistent with a torpedo strike, and seismic data indicated underwater explosions.
14 crew members were rescued, while two were reported missing. A Russian research vessel, Yantar, was suspected of having arrived at the site in early 2025 to potentially recover the cargo.
Other sunken Russian nuclear assets include K-27, a Soviet submarine scuttled in the Kara Sea in 1982, containing nuclear reactors.
K-141 Kursk, a nuclear-powered cruise missile submarine that sank in the Barents Sea in 2000.
This incident remains part of ongoing investigations into potential illicit nuclear cooperation between Russia and North Korea.
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