On a sunny morning in 1966 two US Air Force planes collided and dropped four nuclear bombs near the village of Palomares in southern Spain. There was no nuclear blast, but plutonium was scattered over a wide area - and Spain is now asking the US to finish the clean-up.

The US government calls nukes that go astray "Broken Arrows" and on 17 January 1966, Palomares got four of them.

Overhead, at 31,000ft, an American B-52G bomber collided with a KC-135 tanker plane during routine air-to-air refuelling and broke apart. Three of the bomber's H-bombs landed in or around Palomares, the fourth landed about five miles offshore in the Mediterranean.

Manolo Gonzalez says he was standing outside when he heard a tremendous explosion.

"I looked up and saw this huge ball of fire, falling through the sky," he says. "The two planes were breaking into pieces."

Find out more Watch video of Palomares at the website of PRI's The World, a co-production of the BBC World Service, Public Radio International and WGBH in Boston Palomares and the H-bomb

Gonzalez saw one half of the flaming bomber crash to the ground near the local elementary school - where his wife was teaching.

"I went flying across town on my scooter," he says. "The plane had just barely missed the school itself."

In fact, no-one on the ground was killed that morning. Local people call it the only positive part of this story.

The American airmen weren't so lucky. All four men on the refuelling plane died and three of the seven men on the B-52 were killed (the four others managed to eject safely).

Image caption Navy personnel examine weapon number four, recovered from the sea-bed, 1966

There was only one telephone in Palomares in 1966, and no running water. But the skies over that poor region of southern Spain were being criss-crossed daily by the world's most modern war machines.

It was the height of the Cold War. In an operation code-named Chrome Dome, the US had between 12 and 24 nuclear-armed B-52 bombers in the air 24 hours a day, in an attempt to deter a Soviet first-strike.

There were different flight paths for the B-52s in different parts of the world. The B-52 involved in the Palomares accident was flying the southern route, in a loop from its base in North Carolina around the Mediterranean. The tanker aircraft had taken off from a nearby base in southern Spain to refuel it before the return journey to the US. It was then that disaster struck.

The outcome would have been immeasurably worse if the bombs had been armed. Fortunately they weren't, so there was no nuclear explosion.

In theory, parachutes attached to the bombs should have borne them gently down to earth, preventing any contamination - but two of the parachutes failed to open.

Within days of the crash, the beach in Palomares became a base for a big military operation involving some 700 American airmen and scientists.

The clean-up operation Image caption Air Force men load contaminated dirt into barrels for shipment to the US, 1966 700 US airmen and scientists employed to search for bombs and clean up

Three inches of topsoil removed, sealed in 4,810 barrels and shipped to storage facility in US

20 ships, including mine-sweepers and submersibles, deployed by US Navy to find missing bomb in sea

Cost of sea search over $10m

Yearly health checks thereafter on residents, monitoring of soil, water, air and local crops

Their goal - to find the nukes, and secure them.

The two that fell to earth unsupported by parachutes blew apart on impact, scattering highly toxic, radioactive plutonium dust - a major hazard to anyone who might inhale it.

"What they decided to do was remove the contaminated dirt from the most contaminated areas," says science writer Barbara Moran, author of The Day We Lost the H-Bomb.

They literally scraped up the first three inches of topsoil, sealed it in barrels, and shipped it to a storage facility back in the US.

"They did have a plan in place," Moran says. "But it was supposed to happen on a nice flat piece of ground in the US, not on foreign soil where nobody spoke English and there were all these farmers and goats walking around."

As the clean-up got under way, the US and Spanish governments set out to convince the world there was no danger. US Ambassador Biddle Duke even came down from Madrid for a swim, in front of TV cameras.

When asked by a reporter on the scene if he'd detected any radioactivity in the water, Duke replied with a laugh: "If this is radioactivity, I love it!"

While two of the bombs ruptured on impact, another landed safely. These three were located within 24 hours.

But there was huge consternation about the fourth, which drifted out to sea as it descended, and became known as the "lost" H-bomb.

The US Navy deployed more than 20 ships, including mine-sweepers and submersibles, in an attempt to find it.

Every time the story hits the media it hurts tourism Andres Portillo, local barman

"The design of these bombs was top secret," says Barbara Moran. "When they were searching, there were Soviet spy ships circling around - and the Soviets had submersible technology."

Four months later, as the land clean-up was winding down, the missing bomb was finally hoisted on board a US warship from a depth of 2,850ft (869m). Barbara Moran says the US Navy calculated the total cost of its sea search at over $10m - the most expensive salvage operation in US Navy history to that date.

In Palomares itself, the US and Spain agreed to fund yearly health-checks on residents, and to monitor the soil, the water, the air, and local crops.

Over the years since there's been no evidence that anyone has fallen ill as a result of the accident. The food and water remain clean.

Image caption Tomatoes were an important agricultural product in Palomares in the 1960s

So almost everyone has forgotten about Palomares. Except the people of Palomares. That's because the US clean-up operation missed some areas of contamination. Jose Maria Herrera is a local journalist who's been investigating the accident since the 1980s. He stood recently on a ridge overlooking one of three fenced-off areas which is still contaminated, totalling some 100 acres (40 hectares).

"That crater there is where one of the bombs fell," he says. "You could extract at least half a pound of plutonium from the soil there today."

Actually, just how much plutonium is still out there is hard to determine, because the US has never said how much the bombs were carrying to begin with. But Spanish investigator Carlos Sancho estimates that between 15 and 25 pounds (7 and 11kg) of the material ended up in the soil. Sancho, who runs the Palomares section of the Spanish Department of Energy, insists it does not pose health risks.

The 007 connection In December 1965, a month before the accident, James Bond film Thunderball was released. "The film's plot had strong similarities to what subsequently happened in real life", says author Barbara Moran. "Bond's mission was to find atomic bombs that had been lost at sea. All the news stories at the time were making the connection. "Much of the movie was shot underwater with Sean Connery battling baddies in weird submersibles trying to get the bombs... "In the movie, they had all this really awesome underwater technology that got the bomb. But in real life, it was much harder to first locate, and then recover the bomb from the sea bed."

"The earth there can't be moved because the plutonium is latent in the soil," he says. "If we disturb the soil the plutonium could be dispersed."

So Palomares is like a sleeping dragon. You can't walk in the fenced-off area, and you can't farm it or build on it. The message from the Department of Energy is: "Let the plutonium lie and there's no problem." Yet local people say that in itself is a problem.

Local barman Andres Portillo says the damage is to the town's image. "Every time the story hits the media, it hurts tourism," he says. "A lot of people don't want to come here because they think the quality of life must be low, that cancer rates are higher, when that's not the case at all."

Some here say that without the negative publicity, Palomares could be every bit as popular as its more famous neighbour, Marbella.

So the community finds itself trapped. When residents complain, the accident makes headlines again and there's a drop in the number of visitors, and a drop in the prices farmers get at market for their produce.

But now, 46 years after the accident, there are indications that Spain and the US may be closing in on a permanent solution. Earlier this year, Spain's foreign minister Jose Garcia-Margallo met with US Secretary of State Hillary Clinton, then with reporters.

"Secretary Clinton has said this will be resolved before her mandate is up," Margallo said. "'I am personally committed,' she said."

Though the US State Department quickly released a statement saying that no such commitment had been made, serious talks are under way, says a spokesman for the US embassy in Madrid. As to when an agreement might be reached - over who pays for the second clean-up, how it will be done, and where the contaminated soil will be stored - that's still up in the air.

So the residents of Palomares wait. As they have for nearly half a century. And, from time to time, they allow themselves to dream.

Palomares Deputy Mayor Juan Jose Perez says he hopes he can turn the tragedy into something positive. He'd like to build a museum explaining how it all happened.

"Maybe even in the shape of a B-52 bomber," he says. "We could offer guided walking tours through the affected areas."

But he says for any of that to happen, this story first needs an ending.

For him, a fitting end would be for the US to come back and finish the job.

Additional reporting by Rob Hugh-Jones.

Listen to more on this story at PRI's The World, a co-production of the BBC World Service, Public Radio International, and WGBH in Boston.