Ever since the invention of nuclear weapons, humanity has almost accidentally destroyed itself many times over. This is a video about just some of those times.

It's about nuclear bombs and missiles accidentally detonating. It's about hydrogen bombs being lost. And it's about false alarms that could have led to the end of the world.

On the 17th of January, 1966, a B-52 bomber was flying over the coast of Spain. It was carrying four hydrogen bombs, each one 75 times more powerful than the bomb dropped on Hiroshima.

This sounds crazy, but in the mid 1960s, during the height of the Cold War, this was routine. This was part of a military operation known as Chrome Dome where bombers flew around the United States and skirted the borders of the Soviet Union with hydrogen bombs on board.

The thinking was that if the USSR initiated a first strike, the US could rapidly retaliate by having bombers already up in the air and closer to their targets.

The flight took off from North Carolina and crossed the Atlantic Ocean with the plan to fly by the border of the Soviet Union and return home. The plane would be in the air for more than 24 hours. The flight was so long, it required two mid-air refuelings. But long flights and mid-air refuelings were commonplace. Flights like this happened every day.

The first refuelling went smoothly. After flying by the Soviet Union and turning around, the plane needed to be refuelled again. So at 10:30 a.m., over the coast of Spain, above the small fishing village of Palomares, the refuelling tanker pulled up in front of the B-52. Larry Messinger, the pilot of the bomber, recalled.

- [Larry] We came up behind the tanker and we were going a little bit too fast. There's a procedure they have where the boom operator, if you get in a dangerous position, he calls, "Break away, break away, break away," and you immediately cut the throttles and drop down below. Well, there was no such call. We were dropping down below the tanker and all of a sudden something happened. There was an explosion of some kind.

The two planes collided. The boom arm, which held the refuelling nozzle, hit the B-52, breaking off its left wing. The resulting explosion was big enough that it was witnessed by another B-52 which was flying a kilometre and a half away. All four men on the refuelling tanker and three of the seven on the B-52 were killed in the accident.

The four bombs fell to earth. Each one of them had a yield of 1.1 megatons of TNT, about 75 times more powerful than the bomb dropped on Hiroshima. A hydrogen bomb gets most of its power from the fusion of tritium and deuterium, which are isotopes of hydrogen with extra neutrons. When these isotopes fuse into helium, a little bit of mass is lost, which is released as energy.

But it takes a tremendous amount of energy to get that reaction started, which is why a hydrogen bomb is actually three bombs in one. A conventional bomb which when detonated triggers the plutonium fission bomb which then creates high enough temperatures and pressures and releases enough energetic X-rays to trigger the fusion reaction.

The conventional explosives in two of the four bombs detonated on impact. Fortunately, to trigger the fission reaction, the explosion needs to be symmetrical. But since the conventional explosives detonated on impact with the ground, the shockwave wasn't symmetrical and so the fission and fusion bombs weren't set off. Unfortunately, the conventional explosive blew up the plutonium core, contaminating a 2.6-square-kilometer area of the Spanish coastline with radioactive material. The area is still contaminated with radioactive atoms to this day.

- [Reporter] But after the crash the villagers were told to eat no locally produced food and there was an embargo on it in nearby markets.

The third bomb was found intact in a nearby riverbed. But the fourth bomb was missing.

- [Reporter] Somewhere out there off the sunny coast of southern Spain and H bomb is missing. It's been missing now for over three weeks.

- Mr. Ambassador, how was your swim this morning? Did you by any chance stub your toe on the bomb while you were out there today? - Wish I had.

29 US Army ships were involved in the search, and it still took 81 days for the bomb to finally be located and recovered. I wanna say this again. There was a hydrogen bomb that was 75 times more powerful than the one dropped on Hiroshima, and it was missing for nearly three months.

This is not an isolated incident. It's common enough that the US military has a term for accidents involving nuclear weapons.

- [Reporter] The D-Day clock thus begun ticking with a potential Broken Arrow message from Nellis Air Force Base.

They call them Broken Arrows. The Pentagon officially lists 32 such accidents between 1950 and 1980. Five of them occurred during operation Chrome Dome, cases when a B-52 carrying hydrogen bombs crashed.

In 1961, the first year of operation Chrome Dome, a B-52 had a fuel leak and it crashed over North Carolina. Three members of the crew died in the accident. It was carrying two 4-megaton bombs. Both bombs fell to earth. One had its parachute deploy and it landed on the ground mostly intact. The other slammed into a field and broke into pieces.

The good news is that the conventional explosives did not explode, so radioactive plutonium was not strewn all over North Carolina. The bad news is that the conventional explosives didn't explode, and so there was a chance that the hydrogen bombs could be armed and ready to detonate with full force.

Recalling the incident 52 years later, Lieutenant Jack ReVelle, who was responsible for the safe disposal of the bomb, said.

- [Jack] And as we started digging down, trying to find the second bomb, one of my sergeants says, "Hey, Lieutenant, I found the arm safe switch." And I said, "Great." He says, "No, not great. It's on arm."

At the time, a spokesman for the Department of Defence said that the bombs were unarmed and could not have accidentally exploded. But in a report declassified in 2013, it was stated that the bomb was armed. A single switch could have triggered the detonation. By the way, the bomb is still there, lodged about 50 metres under the field.

After the bomb fell on the field, there was a flash flood, which meant that it couldn't be recovered. So to make sure that no one could dig up the bomb, the US military placed a 120-meter diameter slab of concrete over the whole area. According to the Pentagon report, there is no detectable radiation and no hazard in the area, but a portion of one weapon containing uranium is missing.

There are dozens more events like this, like the plane crash over Greenland in 1968. The four hydrogen bombs it was carrying burned up in the fire. The decontamination of the surrounding area took four months. That same year, the Soviet K-129 submarine sank in the Pacific Ocean, killing all 98 of its crew. It was also carrying three nuclear warheads. Part of the submarine was covertly recovered by the CIA.

Another standout incident occurred in 1965, about 100 kilometres off the coast of Japan. A Douglas A-4 jet carrying at least one hydrogen bomb fell off the aircraft carrier during a training exercise. The plane, the bomb, and the pilot were never recovered. So, for more than half a century, there has been a 1-megaton hydrogen bomb laying on the sea floor four kilometres below the surface of the ocean. It is one of at least six US nuclear weapons that have been lost and never recovered.

In 1998, Alexander Lebed, Russia's former Chief of National Security, asserted that there are more than 100 1-kiloton nuclear bombs that are unaccounted for.

The most well-known Broken Arrow event happened in September 1980. Inside a silo in rural Arkansas, a team was conducting routine checks on the Titan II intercontinental ballistic missile. The missile was fueled, ready to launch at a moment's notice. In its nose cone sat a 9-megaton hydrogen bomb.

As one of the repairmen was walking around the silo, the socket from his ratchet wrench fell off. It fell 24 metres, hitting the fuel tank of the missile and puncturing a hole. The highly flammable rocket fuel began to leak into the silo. The missile complex and surrounding area were evacuated, but a number of military personnel remained on site to try to diffuse the situation.

- The maintenance team had an indication of an engine fire and a rise in vapour concentration inside the silo.

At 3:00 a.m., about 8 1/2 hours after the puncture of the fuel tank, the leaking fuel vapours ignited, resulting in an explosion. This explosion led to one death and 21 injuries. The 740-ton door keeping the silo sealed flew more than 60 metres into the air. It was recovered 200 metres away. The warhead containing the hydrogen bomb was also blown out of the silo, landing some 30 metres away. Luckily, it did not detonate.

And these are just the near misses that we know about. Of the 32 Broken Arrow events listed by the Pentagon, one which occurred in the spring of 1968 remains classified. And there are likely many dozens more nuclear mistakes and near misses the public knows nothing about from every nation that has a nuclear arsenal.

The worry of a nuclear mishap is not just the immediate damage done by a hydrogen bomb explosion. It's also the misunderstanding and retaliatory strikes that could arise. Imagine that in 1966, instead of hydrogen bombs falling off the coast of Spain, the plane crash occurred closer to the border with the Soviet Union.

Humanity has been on the brink of nuclear war many times and so far we've been lucky. The closest we've likely ever come was in October 1962 during the Cuban Missile Crisis. The US Navy began dropping signalling charges into the water to bring a Soviet submarine to the surface. Tensions were high and the submarine had not been in radio contact with Moscow for days.

The captain of the submarine decided that war had broken out and he was about to launch a nuclear torpedo. But the launch required the authorization of three men. Two out of the three authorised the launch, but Vasili Arkhipov did not. A full-blown nuclear exchange between the United States and the Soviet Union was avoided by the rational decision of one person.

The historian Arthur Schlesinger Jr. later noted that "this was not only the most dangerous moment of the Cold War. It was the most dangerous moment in human history."

And during the Cuban Missile Crisis, with the tensions between the US and USSR at an all-time high, both countries still conducted high-altitude nuclear tests. The US set off a 410-kiloton bomb 50 kilometres above the Pacific Ocean and a smaller 7-kiloton bomb at an altitude of 147 kilometres. Two of the 300 kiloton tests conducted by the Soviets occurred in space.

It's pretty hard to justify these actions. What if one of these tests was thought to be a nuclear first strike leading to all-out war? Because false alarms happen for something a lot more innocuous than nuclear tests in space.

On the 26th of September, 1983, the Soviet satellite-based early warning system detected the launch of an intercontinental ballistic missile, and their policy dictated that if any inbound missiles were detected, there was to be an immediate counter attack against the United States. This would've likely led to all-out war.

But the Soviet officer on duty at the time of the detection, Stanislav Petrov, was sceptical of the reading. He reasoned that if there was a genuine first strike, the US would launch hundreds of ICBMs simultaneously, not just one. The detection system then warned Petrov of another four missiles headed towards the USSR, but these two, he dismissed as a glitch.

Petrov was right. He made the right decision and his clear thinking likely saved millions of lives. The warning system malfunctioned, it confused sunlight reflecting off high-altitude clouds for ballistic missiles. After this incident, Petrov took an early retirement and later suffered a nervous breakdown.

Another close call happened on January 25th, 1995, when scientists launched a rocket off the coast of Norway. Their rocket was there to study the Northern Lights but Russian radar picked up this launch as it had a similar flight pattern and speed as the submarine-launched Trident ICBM.

The incident was serious enough that a briefcase containing the nuclear launch codes was taken to the Russian President, Boris Yeltsin.

I think most people believe the biggest threat posed by nuclear weapons is their deliberate use. There is this idea that as long as nations only have them and don't use them, they could act as a deterrent keeping us safer. But all of these incidents demonstrate that the real risk of nuclear weapons is some freak accident.

So far, humanity has been lucky, but how long until our luck runs out? But there is hope. In 1986, there were more than 70,000 nuclear weapons. In 2022, that number had dropped to 12,705, and more are being dismantled. But as long as there are any nuclear weapons remaining, the future of humanity is not safe.

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