Anything that falls into a four-kilometre stretch of a river in the central Peruvian Amazon dies within seconds, because the water reaches temperatures of up to 100 degrees Celsius, despite the river sitting more than 700 kilometres from the nearest active volcano and in a region of the planet with no known magmatic activity

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The Asháninka and Shipibo people, who have lived in the central Peruvian Amazon for many centuries, have a name for it: Shanay-Timpishka. In Quechua, it roughly means boiled with the heat of the sun. It is a section of river, about six kilometres long, in the Mayantuyacu region of the Amazon rainforest. Frogs and insects that fall into it die within seconds. The steam rising from its surface can be seen through the trees from some distance away.

The mud on the banks is too hot to walk on. In some places, the water is genuinely boiling.

Local shamans, who protect the river as a sacred site, believe the heat comes from Yacumama, a great serpent-spirit called the Mother of the Waters. For most of the twentieth century, the outside world — including academic geologists and Peruvian government officials — assumed the story of a boiling river in the Amazon was folklore. There were no volcanoes anywhere near it. There was no known magma. There was, in the standard scientific picture, no possible source of that much heat.

Then, in 2011, a geothermal scientist named Andrés Ruzo went and looked at it.

What he found is one of the strangest hydrothermal features on Earth. Not because it is extremely hot — there are hot rivers in other parts of the world. But because there is essentially no scientific explanation for its existence.

What makes it different from every other hot river

There are geothermal rivers scattered across the planet. Yellowstone has them. Iceland has them. New Zealand’s Waikato River has stretches heated by hot springs. Japan’s Beppu region has entire villages built around geothermally heated water.

All of them have a common feature. They sit on top of active volcanic systems. The heat that raises their water temperature comes from magma — molten rock, usually a few kilometres beneath the surface, warming groundwater that then flows to the surface. This is standard geology and it has been well understood for over a century.

Volcanic geothermal features tend to be small in scale. They come from local hotspots — a specific magmatic system, heating a specific aquifer, producing a specific hot spring or steam vent. Even the largest volcanic hot springs in the world are, ultimately, features of the volcano beneath them.

Shanay-Timpishka sits more than 700 kilometres from the nearest active volcano.

That distance is not slightly too far. It is far outside the range at which any known magmatic system could plausibly heat surface water. The nearest volcanic arc in South America — the Andean chain running down the western spine of the continent — is on the opposite side of a mountain range from the river. Between the Andes and the Amazon there is no magma at any depth that could account for what’s happening at Shanay-Timpishka.

By every conventional understanding of how the Earth heats water, this river should not exist.

What Ruzo actually found

Ruzo, who first heard about the river from his grandfather as a child and had dismissed the story as legend for years, mapped the thermal profile of the entire system beginning in November 2011. What he found is a specific and technical answer to the volcanic puzzle.

The river is heated not by nearby magma, but by ambient geothermal energy — the general warmth of the planet’s crust, which exists everywhere on Earth but is usually far too deep to affect surface water. At Shanay-Timpishka, an unusual combination of geological faults creates a pathway. Rainwater and groundwater sink deep into the Earth through porous sedimentary rock, encounter warm rock at depth, absorb that heat, and are forced back up to the surface through fault-lines that concentrate the flow into a single river channel.

The system is what geologists call a fault-controlled non-volcanic geothermal feature. The individual components — hot rock at depth, water percolation, fault-line resurfacing — are not unusual on their own. The unusual thing is that they combine, at this specific location, at a scale large enough to produce a genuinely hot river four kilometres long. Most similar systems around the world produce, at most, small hot springs of a few metres across. Nothing else on Earth produces a full-scale hot river without volcanic input.

Ruzo’s team measured the hottest single temperature in the river at 99.1 degrees Celsius. That is one-tenth of a degree short of the boiling point of water at sea level. At tropical humidity, and in the specific spots where water emerges from the faults themselves, the river is functionally boiling. The average temperature across the hottest kilometres is around 86 degrees — hot enough to cause...