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Once upon a time, there was a lake that was so radioactive, that standing on its shore for more than an hour would almost definitely kill you. It’s called Lake Karachay, and it’s located in the Ural Mountains in Russia.

In 1990, it was the single most radioactive place on Earth, containing more radioactive material in its small lake bed than the Chernobyl disaster. The lake was contaminated thanks to a long string of bad decisions, which culminated in an explosive failure. But despite all that, we somehow managed to fix it — kind of.

The trouble started way back in 1945 with the top-secret construction of the Mayak nuclear facility in the southern Soviet Union. See, on the other side of the world, the United States had just detonated the first successful atomic bomb in New Mexico. So, not wanting to be shown up, the Soviet Union started their own nuclear project.

They constructed the Mayak plant to manufacture plutonium for their first atomic bomb. By the end of the Cold War, the Mayak facility was home to ten nuclear reactors. It also had several fuel reprocessing plants used to make materials for the Soviet nuclear.

And it's at these plants where things got messy. One of the first steps in reprocessing left behind a radioactive solution of compounds, including sodium nitrate and sodium acetate salts. That solution was left to cool in intermediate storage tanks, and then the remaining mixture.

This mix allowed workers to selectively extract high levels of uranium and plutonium, which could be reused. But it left behind a slurry of other elements, including types of radioactive strontium, caesium, and technetium. Both the strontium and caesium emit high levels of beta rays, a kind of ionizing radiation, as well as a lot of heat.

They also take a while to break down, longer than the facility's storage vats could handle, so they were ultimately considered too hot to store. So what did the plant do? To keep the waste cool, they dumped it in nearby Lake Karachay!

The idea was to hold the waste there until it had decayed enough to go into long-term storage, which, to be clear, was a terrible idea. But for a while, things were okay. After all, Lake Karachay was never intended to be a permanent storage site.

But surprise: That didn’t really work out. Between 1951 and ‘57, because of all the junk being dumped into it, the lake became too radioactive to safely pull the waste back out. And so people just left it.

Now, I wish I could say they realized the error of their ways and stopped there, but this story actually gets worse. In September 1957, the cooling system for one of the intermediate storage tanks, one holding that salt solution, was damaged. And as the temperature in the tanks rose unchecked, the solution began to dry out.

Crystals started to grow out of the drying minerals, similar to how sugar crystals grow when you make rock candy. Only these crystals were made of sodium nitrate and sodium acetate, not sugar. Eventually, as the temperatures reached up to 400°C, the sodium acetate crystals got too hot, and they caught on fire.

To make matters worse, sodium nitrate, although not combustible on its own, is a common accelerant used in explosives. So when the sodium acetate began to combust, the sodium nitrate sped things up. And ultimately, the storage tanks detonated with the equivalent of almost 70 metric tons of TNT, releasing a ton of radioactive materials.

This accident is now considered the third worst nuclear incident in history. Unfortunately, in 1957, the Soviet Union kept it a secret, so this mess wasn’t really cleaned up. Although it did prompt them to stop dumping waste in the lake to avoid investigation.

And over the next ten years, things just kept going downhill. In the 1960s, waste materials from the lake leached into the groundwater used for irrigation and drinking. And in 1967, following a severe drought, Lake Karachay began to dry up from the excessive heat produced by its nuclear waste.

The dry, radioactive lake bed was then swept into the air by the wind and blown across the southern Soviet Union. But Lake Karachay isn’t the most radioactive place in the world anymore. So, how did they fix things?

Well, to start, everyone had to recognize this was a problem. Which they eventually did. Then, in the 1970s and ‘80s, cement blocks were laid across the lake bed to prevent further wind erosion.

Thanks to multi-year funding form the U. S. and European governments, the lake was also completely filled in. And cement was injected into the lake bed to try and stop further groundwater contamination.

By 2015, the lake was completely filled with a special, extra-dense kind of concrete that’s resistant to radiation. And finally, in 2016, a layer of rock and topsoil was laid down, finishing the restoration. Still, despite all the work, we don't recommend relaxing by Lake Karachay any time soon.

Ignoring the radiation concerns, it’s still an active nuclear storage facility. So even if you managed to make it there, the heavily armed guards probably wouldn’t be too pleased with you. If nothing else, though, the lake has been downgraded to something like the seventh most radioactive place on Earth — which is better.

But if we had just disposed of our waste safely, we could have avoided this whole mess. Thanks for watching this episode of SciShow, and to our patrons on Patreon for making it possible! If you’d like to support free online science education and help us make more videos like this, you can go to patreon.com/scishow. ♪.