On 17 January 1966, four atomic bombs fell near Almeria, three on land and one in the sea.
The bombs were being carried by a US Air Force B-52 bomber, which was flying alongside a refuelling aircraft. The two planes collided, killing all four crew members on the refuelling craft and three of the seven on the B-52.
The crash sent the planes hurtling towards the ground from around 31,000 feet - part of the B-52 landed near a school in Palomares, though no residents were harmed that day - and the four nuclear bombs on board the B-52 were released.
The bombs did not have their detonators activated, preventing any explosion. However, the parachutes attached to two of the devices failed to open, meaning they contaminated their crash areas with radioactive plutonium.
A huge US army presence was established in order to recover the bombs and establish the scale of the contamination. The US took away a five-centimetre layer of topsoil from the 25,000 square metres of affected land (or over 1,500 tonnes) and transported it to a storage facility in North Carolina.
The operation to recover the bomb that had fallen in the sea took nearly four months, aided by local fisherman Francisco Simó Orts, who became known as 'Paco el de la bomba'.
Efforts were made to reassure local residents that there was no threat to their health - a famous photo, seen above, shows then-Minister of Tourism Manuel Fraga in the surf near Palomares, alongside US ambassador Angier Biddle Duke.
In reality, however, Palomares is still seriously contaminated as a result of the accident. Experts stated in 1986 that it had the worst plutonium contamination anywhere in the world. How the region would look today had the bombs - 65 times more powerful than the weapon that flattened Hiroshima - detonated does not bear thinking about.
American funds for continual monitoring of the area lasted until 2009. In October 2015, the US agreed to continue the clean-up operation, removing any remaining radioactive waste. However, the agreement was vague, not specifying when the operation would be carried out, nor who will bear its cost.