A lesson buried in the Mediterranean: How a mid-air collision dropped four US nuclear bombs on Spain in 1966

On April 7, 1966, after an exhaustive 80-day search, the US military finally recovered a missing hydrogen bomb from the depths of the Mediterranean Sea. Weighing heavily with a destructive force nearly 100 times that of the atomic bomb dropped on Hiroshima, the warhead was delicately lifted from 2,850 feet (869 meters) below the surface and carefully winched aboard the USS Petrel.

Once safely on deck, officers meticulously dismantled the thermonuclear device to disarm it. Only then did the recovery team breathe a collective sigh of relief — it was the last of four hydrogen bombs accidentally dropped on Spanish soil.

“This was not the first accident involving nuclear weapons,” BBC reporter Chris Brasher said from the scene in 1968. “The Pentagon lists at least nine previous accidents with hydrogen bombs. But this was the first on foreign soil, the first involving civilians, and the first to draw global attention.”

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Operation Chrome Dome and a mid-air disaster

The incident was a consequence of Operation Chrome Dome, a Cold War-era strategy developed by the US in the early 1960s to deter a surprise Soviet attack. Under this program, nuclear-armed B-52 bombers patrolled the skies around the clock, ready to strike Moscow at a moment’s notice. Due to their long routes, these aircraft required frequent mid-air refuelling.

On January 17, 1966, one such B-52 was flying at 31,000 feet (9.5 km) above the Almería region in southern Spain and attempting a routine refuelling with a KC-135 tanker. a US Air Force B-52 bomber collided with a KC-135 tanker during a routine mid-air refuelling mission off the coast of Almería, Spain. The crash claimed the lives of seven out of the eleven crew members aboard both aircraft.

The B-52 was carrying four B28FI Mod 2 Y1 thermonuclear bombs—hydrogen bombs significantly more powerful than those dropped on Hiroshima and Nagasaki. All four warheads were released during the collision and plummeted to Earth. Three landed near the small fishing village of Palomares in southern Spain.

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The impact tore the tanker apart, igniting its fuel and killing all four crew members onboard. The explosion also killed two crew members in the B-52’s tail section. One more crew member ejected but died when his parachute failed. The remaining four managed to escape the burning aircraft before it disintegrated, dropping flaming debris and four thermonuclear bombs over the village of Palomares.

Upon impact, the conventional explosives in two of the bombs detonated, dispersing radioactive plutonium and contaminating approximately 0.77 square miles (2 square kilometers) of land.

The fourth bomb fell into the Mediterranean Sea, triggering an intensive underwater search operation. After a two-and-a-half-month-long effort, the warhead was eventually located and recovered intact.

The search for the missing bomb

Even after recovering three warheads, one bomb remained unaccounted for. “My top priority was recovering the fourth bomb,” Gen Wilson told the BBC.

That night, the Guardia Civil informed the Americans that something resembling a bomb had landed near a local bar — it turned out to be one of the missing warheads discovered in a dry riverbed. Early the next morning, two more bombs were located nearby. But the last one had vanished.

US troops flooded into the area, setting up a base along the Palomares beach. Airmen and scientists began a meticulous search for radiation hotspots, forming long human chains and scanning the area with Geiger counters. Contaminated topsoil — up to three inches deep — was scraped up, sealed in barrels, and shipped to a South Carolina storage facility. Altogether, about 1,400 tonnes of irradiated soil were removed.

Despite a week of intense land searches, the fourth bomb was still missing — until a local fisherman who had helped rescue airmen at sea spoke to Capt Ramirez. The man kept apologising for not saving a flyer he thought had sunk below the waves. Ramirez realised the fisherman had likely seen the bomb, not a person.

Deep-sea recovery

The search shifted offshore. The US Navy launched what would become the most complex ocean recovery operation in history to locate the missing hydrogen bomb, deploying over 100 divers, minesweepers, submarines, and deep-sea submersibles. Nothing of this scale had ever been attempted, but urgency was critical—Soviet ships had begun appearing in the area, raising fears that they might recover the weapon first.

Despite the massive effort, it took months to locate the bomb, which was found more than five miles off the coast. But during an early recovery attempt, it was accidentally dropped back into the sea. A few days later, search teams located it again and successfully retrieved it from the ocean floor. By early April, all four bombs had been accounted for—but the U.S. government’s challenges were far from over.

The long shadow of Palomares

Nearly 60 years later, the legacy of the accident still lingers. Despite an extensive cleanup, not all contaminated areas were properly decontaminated. The US and Spain agreed to conduct annual health checks for Palomares residents and monitor local soil, air, water, and crops.

Still, around 100 acres (40 hectares) of land remain fenced off due to radioactive contamination.

In 2004, a study found that significant radioactive contamination remained in certain areas around Palomares. In response, the Spanish government expropriated several plots of land that would have otherwise been used for agriculture or housing.

On October 11, 2006, Reuters reported that elevated radiation levels had been detected in local snails and other wildlife, suggesting that dangerous amounts of radioactive material might still be present underground.

None of the bombs had exploded, but the plutonium-filled detonators on two went off, spreading several kilograms (pounds) of highly radioactive plutonium 239 across the landscape around Palomares.

Spain and the US signed a statement of intent in 2015 to negotiate a binding agreement to further restore and clear up the Palomares site and arrange for the disposal of the contaminated soil at an appropriate site in the US. But to this day, that promise remains unfulfilled.