Beneath the soaring Art Deco towers of downtown Chicago, its multi-level roads and its busy subway and rail lines, the ground is sinking, and not just for the reasons you might expect.

Since the mid-20th century, the soil between the city surface and the bedrock has warmed by 5.6 degrees Fahrenheit on average, according to a new study from Northwestern University. All that heat, which comes mostly from basements and other underground structures, has caused the layers of sand, clay and rock beneath some buildings to settle or swell by several millimeters over the decades, enough to exacerbate cracks and defects in walls and foundations.

“All around you, you have heat sources,” said the study’s author, Alessandro F. Rotta Loria, walking with a backpack through Millennium Station, a commuter rail terminal under the city’s Loop district. “These are things that people don’t see, so it’s like they don’t exist.”

It’s not just Chicago. In large cities worldwide, human burning of fossil fuels raises mercury at the surface. But heat also pours out of basements, parking lots, train tunnels, pipes, sewers and power cables and into the surrounding land, a phenomenon scientists have called “underground climate change.”

Rising underground temperatures lead to hotter subway tunnels, which can cause overheated tracks and steam bath conditions for commuters. And, over time, they cause small changes in the ground under buildings that can induce structural stress, the effects of which are not noticeable for a long time until suddenly they are.

“Today, you don’t see that problem,” said Asal Bidarmaghz, a senior associate professor of geotechnical engineering at the University of New South Wales in Australia. “But in the next 100 years, there’s a problem. And if we just sit for the next 100 years and wait 100 years to solve it, then that would be a huge problem.”

Dr. Bidarmaghz studied underground heat in London but was not involved in the research in Chicago.

To assess underground climate change in Chicago, Dr. Rotta Loria, assistant professor of civil and environmental engineering at Northwestern, installed more than 150 temperature sensors above and below the surface of the Loop. He combined three years of readings from those sensors with a detailed computer model of the district’s basements, tunnels and other structures to simulate how the soil at different depths warmed between 1951 and now, and how it will warm from now until 2051.

Near some hot springs, the ground beneath Chicagoans’ feet has warmed 27 degrees Fahrenheit over the past seven decades, he found. This caused the clay layers to expand or contract by as much as half an inch under some buildings.

The warming and soil deformation is happening more slowly now than in the 20th century, he found, simply because the earth is closer to being as hot as the basements and tunnels buried in it. Increasingly, these structures will stay warm rather than dissipating heat into the ground around them.

Dr. Rotta Loria’s findings were published Tuesday in the journal Communications Engineering.

The most effective way for building owners and tunnel operators to address the problem, he said, would be to improve insulation so less heat leaks into the ground. They could also turn on the heat. Dr. Rotta Loria is chief technology officer for Enerdrape, a startup in Switzerland making panels that absorb the ambient heat in tunnels and parking garages and use it to power electric heat pumps, cutting down on utility bills. The company installed 200 of its panels in a supermarket parking garage near Lausanne as a pilot project.

Dr. Rotta Loria purposefully did not include one factor in his estimates of underground warming in Chicago: climate change at the city surface.

Warm weather warms the upper layers of soil. But Dr. Rotta Loria’s calculations assume that air temperatures in Chicago remain at their average recent levels until 2051 – that is, his estimates do not include climate scientists’ projections of future global warming. Nor do they account for the fact that, as we continue to warm the planet, large buildings will most likely use more air conditioning and pump even more waste heat into the ground.

The reason for these passes, Dr. Rotta Loria said, is that he is trying to come up with a conservative lower bound on underground warming, not a worst-case scenario. “It already shows that there is a problem,” he said.

The office of Chicago Mayor Brandon Johnson did not respond to requests for comment.

On a recent morning, Dr. Rotta Loria and Anjali Thota, a Northwestern doctoral candidate in civil engineering, took a reporter and a photographer on a tour of their network of temperature sensors that track a kind of invisible city beneath the city.

Dr. Rotta Loria said the Chicago Transit Authority did not allow him to install sensors in subway stations out of concern that people would mistake them for bomb detonators. But he and his team managed to embed sensors in many other familiar and lesser-known places: on commuter rail platforms and at service entrances behind high-rise buildings, in leafy Millennium Park and along Wacker Drive, the cavernous concrete cavern made famous by car chases in the movies “Blues Brothers” and “Dark Knight”.

The sensors themselves are nondescript: a white plastic box with a button and two indicator lights. They cost Dr. Rotta Loria $55 each. The temperature information they collect – one reading every minute or one every 10 minutes, depending on the location – is downloaded to a phone via Bluetooth, which means that Dr. Rotta Loria and his students must periodically visit them in person to harvest their data. around 20,000 discs per day in total.

Many of the sensors have been swiped or disappeared over the years, leaving 100 in service. At Millennium Garages, an underground parking complex, one of them is zip-tied to a pipe behind a pillar.

“That’s all, is it?” said Admir Sefo, an executive at the garage, looking at the window. “And nobody found them?”

“It is difficult even for us to find them,” Ms Thota said. She has their locations saved on Google Maps, but underground, there is often no cell reception, forcing her to hunt.

Another sensor, in the Blackstone hotel, is in a basement room filled with chairs and bags of ice-melting pellets. There is one in the boiler room of the Union League Club of Chicago that has recorded temperatures as high as 96 Fahrenheit. A sensor in the Grant Park South parking garage recorded 97 degrees in September 2021.

Just beyond the walls at each of these places, out of sight and out of mind, this heat silently does what heat does: spread.

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