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When Andrés Ruzo was growing up in Lima, Peru, his grandfather told him an unbelievable story of an ancient lost city of gold hiding deep in the Amazonian rainforest, and the mystical boiling river that adventurers encountered on their journey to discover it. Now, there almost certainly was not an El Dorado waiting undiscovered in the jungle, but even so, something about the story stuck with little Andrés up until the point that he became Dr. Ruzo.

When some of Ruzo's colleagues in the Peruvian government reached out to him to review a set of maps that were being prepared for publishing, he couldn't help but notice a series of hot springs located in the jungle — more than you would expect for a place not known for its volcanic activity. Although others in his profession disparaged the idea as a fool's errand (one even pleaded with him to stop asking stupid questions), he resolved to settle the matter once and for all. And into the jungle he went.

You saw the headline, so you can probably guess how his journey ended. The Boiling River doesn't quite live up to its name, but it does hover around 200 degrees Fahrenheit (93 degrees Celsius), with some variation throughout the day. That's hot enough to fill the air with steam, to kill any animals unfortunate enough to fall in, and to become a major part of the spiritual lives of the locals. It's really not hard to see why it became cemented into stories of mythic adventure and lost cities.

What is hard to say is why it's so hot. One theory was that it could have been a volcanic feature, a result of previously unnoticed magma flows — but that was a non-starter since, well, there weren't any magma flows. Another hypothesis was that it was the result of a disaster. If one of the nearby oil companies had accidentally set off a chain reaction underground, it could have started something like this. That wasn't possible either since descriptions of the Boiling River can be found in Spanish records dating back hundreds of years. The only other option is that it's a non-volcanic feature, the result of water flowing from deep, deep underground where the temperature is naturally much hotter. As far as explanations go, it's not exactly satisfying. But it's a whole lot better than a centuries-old oil spill.

We've only scratched the surface of Andrés Ruzo's journey into the jungle. Check out his book, "The Boiling River" for the full story. We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.