When the Ursa Major sank between Spain and Algeria on December 23, 2024, Russia’s Foreign Ministry called it an engine room explosion. No explanation for the blast. No elaboration.

Fourteen of the ship’s sixteen crew members were rescued and brought to Spain. Second engineer Nikitin and engineer Yakovlev remain missing, presumed dead.

Seventeen months later, the story has changed entirely. Spanish investigators have concluded that the vessel was carrying components for two submarine nuclear reactors — and that its ultimate destination was not Vladivostok, as the manifest claimed, but the North Korean port of Rason.

The only reason any of this came to light is that the ship went down.

A Manifest Built to Deceive

The Ursa Major, previously known as Sparta III, was constructed in 2009 and operated by Oboronlogistika, a company owned by the Russian Ministry of Defence. Its official manifest described a voyage from St Petersburg to Vladivostok carrying two large cranes, 129 empty containers, and two hatch covers classified as “non-dangerous merchandise.”

The hatch covers were reactor components. The cranes, investigators believe, were included to assist with unloading the nuclear cargo once it reached North Korea. The empty containers were window dressing. And the Vladivostok destination was almost certainly a fiction — investigators have noted that Russia maintains an extensive rail network to its own Far East, raising the obvious question of why such cargo would travel by sea around the entire globe unless the real destination required a maritime route.

The ship’s Russian captain confirmed the deception. He told investigators that items declared on the manifest as hatch covers were actually components for two nuclear reactors similar to those used in submarines, according to a source familiar with the investigation who spoke to CNN. The captain said he believed the ship would eventually be diverted to Rason. He declined to elaborate further, citing fears for his safety.

Soldiers for Reactors

The timeline tells its own story. The sinking occurred just two months after North Korean leader Kim Jong-un dispatched roughly 10,000 troops to support Russia’s full-scale invasion of Ukraine. Russia and North Korea also have a mutual military aid pact in place since late 2024. Investigators believe the nuclear transfer may have been the Kremlin’s side of the bargain.

A nuclear-powered submarine has been on Pyongyang’s wish list since at least 2021, when Kim used a political conference to outline a slate of desired weapons systems that also included solid-fuelled intercontinental ballistic missiles and hypersonic weapons. In December 2025, North Korea released photographs of Kim inspecting what appeared to be a largely completed nuclear-powered submarine hull under construction, coated with what analysts identified as anti-corrosion paint.

Pyongyang has said it plans to arm the submarine with nuclear weapons, calling it a “strategic nuclear attack submarine.” For years, the question was how a heavily sanctioned state could acquire the reactor technology to build one. The Ursa Major appears to have been part of the answer.

South Korean intelligence reported in September 2025 that Moscow had already delivered one nuclear reactor to Pyongyang. Multiple South Korean government officials told domestic media the Kremlin was suspected of sending two to three nuclear submarine propulsion modules to North Korea in the first half of 2025 alone.

Sabotage on the Seabed

The cause of the sinking remains contested. Spanish investigators believe the Ursa Major may have been deliberately sunk by a Western military using a rare supercavitating torpedo — a weapon designed to travel at extreme speed underwater — to prevent the delivery of advanced nuclear technology to North Korea, according to details of the probe obtained by CNN.

No government has claimed responsibility.

Whatever happened at sea that December night, the aftermath has been revealing. The Russian spy vessel Yantar spent five days positioned directly over the Ursa Major’s wreckage, which lies at a depth of approximately 2,500 meters. US nuclear “sniffer” aircraft have flown over the site twice in the past year, according to public flight data. Spanish authorities have said that recovering the ship’s data recorder would be prohibitively expensive and dangerous — a position that experts have questioned, given that it makes little sense if no radioactive material was involved.

The Syria Distraction

The Syria angle, widely reported at the time, now appears to have been cover. Ukrainian military intelligence reported that the Ursa Major was en route to assist with evacuating Russian military equipment from the Tartus and Khmeimim bases following the fall of Bashar al-Assad’s regime. The vessel had been part of Russia’s “Syrian Express” supply route for years, and a UK sanctions listing notes that Oboronlogistika ships have been used to transport missiles from Syria to the Black Sea.

The Spanish investigation concluded that the Syria indications were “likely a distraction from the trip’s true purpose.”

The Pipeline That Wasn’t Caught

Oboronlogistika had announced just two months before the sinking that its ships were licensed to carry nuclear material. The Ursa Major had been under US and UK sanctions since Russia’s 2022 invasion of Ukraine. Neither the sanctions regime nor the international monitoring architecture designed to track nuclear proliferation detected what was happening.

The ship loaded reactor components in St Petersburg, sailed west through the Mediterranean, and was intercepted only by catastrophe. Every system meant to prevent this transfer failed — and the only reason the world learned what was on board is that the Ursa Major never completed its voyage.

That leaves a question no investigator has answered: how many others made it through?

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