Atomic bombs are some of the most dangerous weapons humans have ever made.

They can destroy whole cities and kill tens of thousands of people in seconds, leaving countless more with radiation-related illnesses for the rest of their lives. But humanity didn’t stop with atomic bombs.

After those came fusion bombs, which use the energy from a regular atom bomb to trigger an even bigger explosion. And those still aren’t the worst types of nukes people have ever imagined. That award might go to salted nuclear weapons, which would intentionally spray extra radiation out into the world.

They’ve never been tested or built, as far as anyone knows -- or admits. But if you want to wipe out humanity, they’re probably your best shot. Salted nukes are named that way because they’re like a typical fusion bomb, except with extra stuff added -- sort of like adding salt to a pretty weird meal.

They were first described back in 1950 by physicist Leo Szilard, who worked on the first nukes just a few years earlier. Szilard wanted to convince people that nukes should be destroyed instead of stockpiled, and to drive the point home, he imagined a worst-case scenario: A nuke that could seriously threaten all life on Earth. They would work by utilizing the leftover neutrons from a regular fusion bomb to create extra radiation.

See, the fusion bombs being developed at the time -- also known as hydrogen or thermonuclear bombs -- would use the energy from an atomic bomb to fuse types of hydrogen into helium and create huge explosions. Those explosions also released lots of extra neutrons, which weren’t used up in the reactions that caused the explosion. Fusion bombs were the biggest bombs ever made, but even before the first one blew up, Szilard knew there’d be a way of making them even more dangerous.

If certain kinds of extra atoms were put into a fusion bomb, some of those unused neutrons would crash into them during the explosion. That would add an extra neutron to those atoms and make them radioactive. Then, the explosion would send the radioactive atoms into the environment or up into the atmosphere, where they’d send out radiation as they decayed.

Those extra atoms are the so-called “salt” of a salted nuke. Most nukes release some radioactive atoms in the explosion, but they’re usually necessary byproducts of the explosion itself. For example, a kind of cesium is released because it comes from unstable versions of atoms like uranium, which are used to build the bomb.

The salt would be different. It wouldn’t do anything in the explosion itself. Its whole purpose would be to spread extra radiation and make the effects of the bomb deadlier, longer-lasting, and wider-spread.

So… not great. Now, some radioactive atoms decay pretty quickly. So if they were the salt, they wouldn’t spread too far before just about all of them decayed and released their radiation.

After that, they wouldn’t really be dangerous anymore. Other types of “salt” could have much wider effects. Salted nukes are sometimes known as “cobalt bombs”, because nukes salted with cobalt were Szilard’s original worst-case scenario.

The isotope cobalt-60 -- which is a version of cobalt with an extra neutron -- has a half-life of a little more than five years. So half a sample of cobalt-60 will decay and release its radiation every five years or so. Enough cobalt could take so long to fully decay that the area around the explosion would be uninhabitable for almost a century.

But, that’s not the only problem. Since it takes so long to break down, Szilard pointed out that there’s plenty of time for radioactive cobalt-60 from a salted nuke to get swept by air currents and spread around the world. Small amounts of it would also stay in the atmosphere worldwide for decades — because once it’s out there, there’s no real way of getting it back.

Szilard originally estimated that it wouldn’t take many cobalt bombs for their cumulative radiation to seriously threaten all life on Earth. Now, today’s experts think that’s a little extreme. Modern nukes are generally smaller than the ones imagined in the 50s, and it would take a lot of cobalt to spread around the entire planet.

Plus, like, deep-sea bacteria are probably going to be just fine. But a volley of cobalt bombs would definitely be incredibly dangerous, whether or not you’re near the explosions and whether they happened yesterday or ten years ago. If all this sounds a little familiar, it might be because salted nukes are the doomsday device in the 1960s movie Dr.

Strangeglove. But unlike the movie, no country on Earth has ever officially built or tested a salted nuke -- at least, as far as anyone can tell. And as we’ve seen in the last couple years, it’s kinda hard to test a nuke without someone noticing.

They wouldn’t be impossible for a nuclear-armed country to build, but publicly, everyone pretty much agrees that these things are horrible. And that there are more efficient ways of blowing stuff up. Still, there were headlines a few years ago after what seemed like an accidental leak of Russian plans involving one.

The plans were for a gigantic salted nuke that would release huge waves of radioactive water toward the United States. But experts have dismissed the leak as propaganda. It required exploding a bomb underwater -- one twice as large as the largest bomb ever exploded -- to create a completely uncontrollable tsunami.

Which seemed impractical. And everyone else in the world super promises they don’t have any salted nukes, either. Which feels like a relief.

Because, let’s be real: Regular ol’ nukes are bad enough. Thanks for watching this episode of SciShow! If you’d like to learn more about nuclear weapons -- like how you’d stop one -- you can watch one of our other episodes and keep getting smarter with us.