Every once in a while, a high reading of radioactivity off the coast of Tybee Island, Georgia, sends the US government scrambling to look for a nuclear weapon that's likely hidden 13 to 55 feet below the ocean and sand, buried in the seafloor.

On February 5, 1958, two Air Force jets collided in mid-air during a training mission. The B-47 strategic bomber carried a Mark 15 thermonuclear bomb.

For over two months, the Air Force and Navy divers searched a 24-square-mile area in the Wassaw Sound, a bay of the Atlantic Ocean near Savannah. They never found the nuclear bomb.

Forty years later, a retired Air Force officer who remembered newspaper stories about the lost bomb from his childhood started a search for it.

"It's this legacy of the Cold War," said Stephen Schwartz, author of "Atomic Audit: The Costs and Consequences of US Nuclear Weapons Since 1940."

"This is kind of hanging out there as a reminder of how untidy things were and how dangerous things were."

But some experts say that even if someone finds the bomb, it may be better to leave it buried.

An armed training mission

At the time of the collision, it was "common practice" for the Air Force pilots on training missions to carry bombs on board, according to a 2001 report about the Tybee accident.

The purpose of the training mission was to simulate a nuclear attack on the Soviet Union. They practiced flying over different US cities and towns to see whether the electronic beam would reach its target.

Major Howard Richardson, flying a B-47 carrying the weapon, completed his mission. Meanwhile, another pilot, Lieutenant Clarence Stewart, was on his own training mission in an F-86 to intercept the jets. But Stewart's radar didn't pick up that there were two B-47s, and he and Richardson collided.

Everyone survived the crash. Stewart ejected and got frostbite. Richardson realized he couldn't land his damaged plane on the Air Force base's under-construction runway with the weight of the weapon.

He headed for the ocean, dropped the nuclear bomb from about 7,200 feet, and landed the B-47 safely.

The plane's crew didn't see an explosion afterward, according to the 2001 report. But in 2008, Richardson wrote in a Savannah Morning News article that he and the passengers may not have seen the bomb go off because he'd turned the plane.

In 2004, Richardson told CBS News he regretted dropping the bomb because of all the trouble it caused.

"What I should be remembered for is landing that plane safely," he said. "I guess this bomb is what I'm going to be remembered for."

The question of the plutonium capsule

For weeks after the collision, about 100 Navy divers searched for the weapon using handheld sonar. Blimps and ships scoured the coast and marshes, the Atlanta Constitution reported at the time.

On April 16, 1958, the military decided the bomb was "irretrievably lost." At the time, the Air Force said the weapon wasn't fully assembled and "there was no danger of an explosion or radioactivity," the Atlanta Constitution reported.

Back then, the technology hadn't progressed to sealed nuclear weapons. Instead, the plutonium was separate from the bomb casing and the explosives that caused the implosion, Schwartz said. The weapon was only "complete" when the plutonium capsule or core was inside.

"Only when it was complete could it be armed and set off and achieve a nuclear chain reaction," Schwartz said.

The US government and military have repeatedly said the Tybee weapon didn't contain a plutonium capsule when Richardson jettisoned it. A receipt for the bomb that Richardson signed at the time said he wouldn't allow the insertion of an "active capsule" into the weapon.

A 1966 letter declassified in 1994 complicated the picture. It referred to then-Assistant Defense Secretary Jack Howard's testimony before a congressional committee calling the Tybee bomb a complete nuclear weapon, with plutonium included. In 2001, a military spokesman told The Atlantic that they had recently spoken with Howard, and "he agreed that his memo was in error."

"I know some people think it's settled," Schwartz said. "I haven't fully made up my mind, but I feel like I am more comfortable going with the contemporaneous accounts."

Detecting a bomb underwater

In 2000, retired Air Force officer Derek Duke contacted then-Rep. Jack Kingston of Georgia about the missing bomb and Howard's memo, CBS News reported. At the congressman's urging, the Air Force looked into the pros and cons of trying to locate and remove the weapon versus leaving it alone.

The 2001 report suggested recovery cost would start at $5 million, and that there was "a very low possibility of successfully locating the bomb." There was little chance it would spontaneously explode, it didn't contain plutonium, and the biggest environmental risk was heavy metal contamination as the bomb corroded, the report concluded.

But there was a chance of it exploding during retrieval, and experts would have to remove and dispose of the uranium first.

"The whole Air Force perspective is, it's just not worth it," Schwartz said. "Trying to move it could create bigger problems than if we just leave it where it is."

The 2001 Air Force report estimated the 7,600-pound lost bomb had 400 pounds of conventional explosive.

Nevertheless, Duke took it upon himself to find the weapon. In 2004, he thought he had it. His equipment picked up unusually high radiation readings.

The Air Force investigated but reported that the radiation was from naturally occurring minerals in Wassaw Sound.

Over a decade later, in 2015, another citizen found strange sonar readings. The Nuclear Emergency Support Team advised on Operation Sleeping Dog, when military divers again searched for and failed to find the nearly 12-foot-long bomb.

The Department of Energy sent subject matter experts to examine what the citizen searchers found in 2015, Shayela Hassan, deputy director of the Office of Communications with the National Nuclear Security Administration, said in an emailed statement to Insider.

"DOE's assessment of the material presented in 2015 was that the search lacked any evidence that supported discovery of the lost weapon," the statement said.

The agency statement continued: "Periodic announcements by private citizens that the bomb may have been located have prompted mobilizations of US Government personnel, diverting them from more pressing national security and public health responsibilities. As such, DOE does not encourage private searches for the device."

Schwartz thinks the only way the weapon will be found is by chance or if a powerful storm dredges it up.

"I won't say it's lost for the ages because I don't think it is," he said, but "so many people have searched for it for so long using some fairly sophisticated equipment and not found it."

One mishap among many

Less than a month after Richardson jettisoned the Tybee bomb, another B-47 accidentally dropped a nuclear weapon on South Carolina. It didn't contain plutonium but left a 50-foot crater in a family's yard. A few family members had minor injuries but everyone survived.

Since 1950, the US military has been involved in 32 "broken arrow" incidents, where they lost or dropped nuclear weapons or other issues, like fires, were involved.

In his book "Command and Control," Eric Schlosser wrote that in 1957 Air Force planes unintentionally dropped a nuclear weapon once every 320 flights. Coupled with the high rate of B-52 bomber crashes, there was the potential for about 19 incidents involving nuclear weapons each year.

Between 1960 and 1968, the US military kept jets armed with nuclear weapons at the ready in case of a surprise nuclear attack. A series of near misses and serious accidents with nuclear weapons caused the Air Force to end the program.

"I don't think we're going to go back to the bad old days of putting our nuclear weapons on aircraft," Schwartz said.

(In 2007, a B-52 bomber was accidentally loaded with six cruise missiles carrying nuclear warheads and transported without safety precautions, a mistake that would lead to the resignations of the Air Force secretary and chief of staff.)

But Schwartz thinks incidents like Tybee — whether or not it contained plutonium — can remind people about the narrow misses with nuclear disasters.

"To have this many accidents and not have a weapon accidentally detonate is not just luck. It's also good engineering," he said. "But we also got incredibly lucky."

This article was originally published by Business Insider.

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