The B-52G began its mission from Seymour Johnson Air Force Base, North Carolina, carrying four type B28RI hydrogen bombs on a Cold War airborne alert mission named Operation Chrome Dome. The flight plan took the aircraft east across the Atlantic Ocean and Mediterranean Sea towards the European borders of the Soviet Union before returning home. The lengthy flight required two mid-air refuellings over Spain.
At about 10:30 am on 17 January 1966, while flying at 31,000 feet (9,450 m), the bomber commenced its second aerial refuelling with a KC-135 out of Morón Air Base in southern Spain. The B-52 pilot, Major Larry G. Messinger, later recalled,
"We came in behind the tanker, and we were a little bit fast, and we started to overrun him a little bit. There is a procedure they have in refueling where if the boom operator feels that you're getting too close and it's a dangerous situation, he will call, 'Break away, break away, break away.' There was no call for a break away, so we didn't see anything dangerous about the situation. But all of a sudden, all hell seemed to break loose."
The planes collided, with the nozzle of the refueling boom striking the top of the B-52 fuselage, breaking a longeron and snapping off the left wing, which resulted in an explosion that was witnessed by a second B-52 about a mile away. All four men on the KC-135 and three of the seven men on the bomber were killed.
Those killed in the tanker were boom operator Master sergeant Lloyd Potolicchio, pilot Major Emil J. Chapla, copilot Captain Paul R. Lane, and navigator Captain Leo E. Simmons.
On board the bomber, navigator First Lieutenant Steven G. Montanus, electronic warfare officer First Lieutenant George J. Glessner, and gunner Technical Sergeant Ronald P. Snyder were killed. Montanus was seated on the lower deck of the main cockpit and was able to eject from the plane, but his parachute never opened. Glessner and Snyder were on the upper deck, near the point where the refuelling boom struck the fuselage, and were not able to eject.
Four of the seven crew members of the bomber managed to parachute to safety: Major Messinger, aircraft commander Captain Charles F. Wendorf, copilot First Lieutenant Michael J. Rooney and radar-navigator Captain Ivens Buchanan. Buchanan received burns from the explosion and was unable to separate himself from his ejection seat, but he was nevertheless able to open his parachute, and he survived the impact with the ground. The other three surviving crew members landed safely several miles out to sea.
The Palomares residents carried Buchanan to a local clinic, while Wendorf and Rooney were picked up at sea by the fishing boat Dorita. The last to be rescued was Messinger, who spent 45 minutes in the water before he was brought aboard the fishing boat Agustin y Rosa by Francisco Simó Orts. All three men who landed in the sea were taken to a hospital in Águilas.
The aircraft and hydrogen bombs fell to earth near the fishing village of Palomares. This settlement is part of Cuevas del Almanzora municipality, in the Almeria province of Andalucía, Spain. Three of the weapons were located on land within 24 hours of the accident—the conventional explosives in two had exploded on impact, spreading radioactive contamination, while a third was found relatively intact in a riverbed. The fourth weapon could not be found despite an intensive search of the area—the only part that was recovered was the parachute tail plate, leading searchers to postulate that the weapon's parachute had deployed, and that the wind had carried it out to sea.
During early stages of recovery after the accident the 66th Tactical Reconnaissance Wing, flying RF-101C Voodoos out of RAF Upper Heyford near Oxford, England, provided aerial photographs to assist in the recovery operation and to document the crash site.
On 22 January, the Air Force contacted the U.S. Navy for assistance. The Navy convened a Technical Advisory Group (TAG), chaired by Rear Admiral L. V. Swanson with Dr. John P. Craven and Captain Willard Franklyn Searle, to identify resources and skilled personnel that needed to be moved to Spain.
The search for the fourth bomb was carried out by means of a novel mathematical method, Bayesian search theory, led by Dr. Craven. This method assigns probabilities to individual map grid squares, then updates these as the search progresses. Initial probability input is required for the grid squares, and these probabilities made use of the fact that a local fisherman, Francisco Simó Orts, popularly known since then as "Paco el de la bomba" ("Bomb Paco" or "Bomb Frankie"), witnessed the bomb entering the water at a certain location. Orts was hired by the U.S. Air Force to assist in the search operation.
The United States Navy assembled the following ships in response to Air Force request for assistance:
Additionally, the aircraft carrier USS Forrestal and various other units of the Sixth Fleet made a brief stopover at Palomares on the morning of March 15, 1966, with Forrestal anchoring at 0903 and departing at 1219.
The recovery operation was led by Supervisor of Salvage, Capt Searle. Hoist, Petrel and Tringa brought 150 qualified divers who searched to 120 feet (37 m) with compressed air, to 210 feet (64 m) with mixed gas, and to 350 feet (110 m) with hard-hat rigs; but the bomb lay in an uncharted area of the Rio Almanzora canyon on a 70-degree slope at a depth of 2,550 feet (780 m). After a search that continued for 80 days following the crash, the bomb was located by the DSV Alvin on 17 March, but was dropped and temporarily lost when the Navy attempted to bring it to the surface. After the loss of the recovered bomb the ship's positions were fixed by Decca HI-FIX position-locating equipment for subsequent recovery attempts.
Alvin located the bomb again on 2 April, this time at a depth of 2,900 feet (880 m). On 7 April, an unmanned torpedo recovery vehicle, CURV-I, became entangled in the weapon's parachute while attempting to attach a line to it. A decision was made to raise CURV and the weapon together to a depth of 100 feet (30 m), where divers attached cables to them. The bomb was brought to the surface by USS Petrel. The USS Cascade was diverted from its Naples destination and stayed on scene until recovery and took the bomb back to America.
Once the bomb was located, Simó Orts appeared at the First District Federal Court in New York City with his lawyer, Herbert Brownell, formerly Attorney general of the United States under President Dwight D. Eisenhower, claiming salvage rights on the recovered hydrogen bomb. According to Craven:
"It is customary maritime law that the person who identifies the location of a ship to be salved has the right to a salvage award if that identification leads to a successful recovery. The amount is nominal, usually 1 or 2 percent, sometimes a bit more, of the intrinsic value to the owner of the thing salved. But the thing salved off Palomares was a hydrogen bomb, the same bomb valued by no less an authority than the Secretary of Defense at $2 billion—each percent of which is, of course, $20 million."
The Air Force settled out of court for an undisclosed sum. In later years, Simó was heard to complain that the Americans had promised him financial compensation and had not kept that promise.
At 10:40 a.m. UTC, the accident was reported at the Command Post of the Sixteenth Air Force, and it was confirmed at 11:22. The commander of the U.S. Air Force at Torrejon Air Base, Spain, Major General Delmar E. Wilson, immediately traveled to the scene of the accident with a Disaster Control Team. Further Air Force personnel were dispatched later the same day, including nuclear experts from U.S. government laboratories.
The first weapon to be discovered was found nearly intact. However, the conventional explosives from the other two bombs that fell on land detonated without setting off a nuclear explosion (akin to a dirty bomb explosion). This ignited the pyrophoric plutonium, producing a cloud that was dispersed by a 30-knot (56 km/h; 35 mph) wind. A total of 260 ha (2.6 square kilometres (1.0 sq mi)) was contaminated with radioactive material. This included residential areas, farmland (especially tomato farms) and woods.
To defuse alarm of contamination, on 8 March the Spanish minister for information and tourism Manuel Fraga Iribarne and the United States ambassador Angier Biddle Duke swam on nearby beaches in front of press. First the ambassador and some companions swam at Mojácar — a resort 15 km (9 mi) away — and then Duke and Fraga swam at the Quitapellejos beach in Palomares.
Despite the cost and number of personnel involved in the cleanup, forty years later there remained traces of the contamination. Snails have been observed with unusual levels of radioactivity. Additional tracts of land have also been appropriated for testing and further cleanup. However, no indication of health issues has been discovered among the local population in Palomares.
President Lyndon B. Johnson was first apprised of the situation in his morning briefing the same day as the accident. He was told that the 16th Nuclear Disaster Team had been sent to investigate, per the standard procedures for this type of accident. News stories related to the crash began to appear the following day, and it achieved front page status in both the New York Times and Washington Post on 20 January. Reporters sent to the accident scene covered angry demonstrations by the local residents. On 4 February, an underground Communist organization successfully initiated a protest by 600 people in front of the U.S. Embassy in Spain. The Duchess of Medina Sidonia, Luisa Isabel Álvarez de Toledo (known as the "Red Duchess" for her socialist activism), eventually received a 13-month prison sentence for leading an illegal protest.
Four days after the accident, the Spanish government stated that "the Palomares incident was evidence of the dangers created by NATO's use of the Gibraltar airstrip", announcing that NATO aircraft would no longer be permitted to fly over Spanish territory either to or from Gibraltar. On 25 January, as a diplomatic concession, the U.S. announced that it would no longer fly over Spain with nuclear weapons, and on 29 January the Spanish government formally banned U.S. flights over its territory that carried such weapons. This caused other nations hosting U.S. forces to review their policies, with the Philippine Foreign Secretary Narciso Ramos calling for a new treaty to restrict the operation of U.S. military aircraft in Filipino airspace.
Palomares, and the Thule Air Base B-52 crash involving nuclear weapons two years later in Greenland, made Operation Chrome Dome politically untenable, leading the U.S. Department of Defense to announce that it would be "re-examining the military need" for continuing the program.
Fifty years later in the town of Palomares, most people prefer to forget the incident, and it is now noted only by a street named "17 January 1966".
Soil with radioactive contamination levels above 1.2 MBq/m2 was placed in 250-litre (66 U.S. gallon) drums and shipped to the Savannah River Plant in South Carolina for burial. A total of 2.2 hectares (5.4 acres) was decontaminated by this technique, producing 6,000 barrels. 17 hectares (42 acres) of land with lower levels of contamination were mixed to a depth of 30 centimetres (12 in) by harrowing and plowing. On rocky slopes with contamination above 120 kBq/m2, the soil was removed with hand tools and shipped to the United States in barrels.
In 2004, a study revealed that there was still some significant contamination present in certain areas, and the Spanish government subsequently expropriated some plots of land which would otherwise have been slated for agriculture use or housing construction. In early October 2006, the Spanish and United States governments agreed to decontaminate the remaining areas and share the workload and costs, which are hitherto unknown, as it first needs to be determined to what extent leaching of the plutonium has occurred in the 40 years since the incident.
On 11 October 2006, Reuters reported that higher than normal levels of radiation were detected in snails and other wildlife in the region, indicating there may still be dangerous amounts of radioactive material underground. The discovery occurred during an investigation being carried out by Spain's energy research agency CIEMAT and the U.S. Department of Energy. The U.S. and Spain agreed to share the cost of the initial investigation.
In April 2008, CIEMAT announced they had found two trenches, totalling 2,000 cubic metres (71,000 cu ft), where the U.S. Army stored contaminated earth during the 1966 operations. The American government agreed in 2004 to pay for the decontamination of the grounds, and the cost of the removal and transportation of the contaminated earth has been estimated at $2 million. The trenches were found near the cemetery, where one of the nuclear devices was retrieved in 1966, and they were probably dug at the last moment by American troops before leaving Palomares. CIEMAT expects to find remains of plutonium and americium once an exhaustive analysis of the earth is carried out. In a conversation in December 2009, the Spanish Foreign Minister Miguel Ángel Moratinos told the US Secretary of State Hillary Clinton that he feared Spanish public opinion might turn against the US once the results of the study on nuclear contamination were to be revealed.
In August 2010, a Spanish government source revealed that the U.S. has stopped the annual payments it has made to Spain, as the bilateral agreement in force since the accident expired the previous year.
On 19 October 2015 Spain and the United States signed an agreement to further discuss the cleanup and removal of land contaminated with radioactivity. Under a statement of intent signed by Spanish Foreign Minister José García-Margallo y Marfil and U.S. Secretary of State John Kerry, the two countries will 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 United States.
The empty casings of two of the bombs involved in this incident are now on display in the National Museum of Nuclear Science & History in Albuquerque, New Mexico.
While serving on the salvage ship USS Hoist during recovery operations, Navy diver Carl Brashear had his leg crushed in a deck accident and lost the lower part of his left leg. His story was the inspiration for the 2000 film Men of Honor.
In March 2009, Time magazine identified the Palomares accident as one of the world's "worst nuclear disasters".
There has been a marked long-term occurrence of cancer and other health defects among the surviving USAF personnel who were directed to the site in the days following the accident to clean up the contamination. Most of the afflicted personnel have had difficulty securing any type of compensation from the Department of Veterans Affairs due to the secretive nature of the cleanup operation and the Air Force's refusal to acknowledge that adequate safety measures to protect the first responders may not have been taken.
In June 2016, The New York Times published an article on the 50th anniversary lingering legacy of the Palomares accident.