There's a "silent hazard" threatening the future of major cities. A new study found that the ground underneath major cities is heating up so much that it's becoming deformed – and that buildings, as they are, likely won't be able to handle it as it gets worse.
The study was conducted by researchers at Northwestern University, who used Chicago as a "living laboratory" to research the impact that underground temperature variations have on infrastructure.
"The ground is deforming as a result of temperature variations, and no existing civil structure or infrastructure is designed to withstand these variations," researcher and Northwestern professor Alessandro Rotta Loria said in a press release. "Although this phenomenon is not dangerous for people's safety necessarily, it will affect the normal day-to-day operations of foundation systems and civil infrastructure at large."
The problem is something called "underground climate change," otherwise known as "subsurface heat islands." It's a phenomenon that, along with threatening infrastructure, can lead to contaminated groundwater and impact health conditions such as asthma.
It's been minimally researched, so Rotta Loria and his team installed more than 150 temperature sensors above and below ground the Chicago Loop to learn more. Those sensors were put in basements, subway tunnels and buried under Grant Park along Lake Michigan, among other areas.
What they found is that underground temperatures in this loop are often 10 degrees Celsius warmer than those beneath Grant Park. Air temperatures vary even more – getting up to 25 degrees Celsius warmer compared to undisturbed ground temperatures.
Rotta Loria told CBS News that there is a "myriad of heat sources" underground that contribute to the warming, including basements, parking garages and subway tunnels.
"This is significant because it is renowned that materials such as soils, rocks and concrete deform when subjected to temperature variations," Rotta Loria said of his research, which was published July 11 in Communications Engineering, a Nature Portfolio journal.
And it isn't just happening in Chicago.
"We used Chicago as a living laboratory, but underground climate change is common to nearly all dense urban areas worldwide," Rotta Loria said in a Northwestern press release. "And all urban areas suffering from underground climate change are prone to have problems with infrastructure."
In Chicago, the ground is filled with clay, which Rotta Loria says can contract as temperatures increase, just as what happens with other types of soil. So as the temperatures increase, it's causing building foundations in the city to undergo "unwanted settlement, slowly but continuously."
"Underground climate change is a silent hazard," he said. "... In other words, you don't need to live in Venice to live in a city that is sinking – even if the causes for such phenomena are completely different."
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So why is all this happening?
"Global warming definitely plays a role in all of this," Rotta Loria said. "It is renowned that the temperature in the ground is linked to the temperature that we find at the surface of cities. So as the temperature above the ground is rising, also the temperature underground rises."
Parts of cities have been known to be up to 20 degrees Fahrenheit warmer than other spots just a few miles away because of the urban heat island effect. This effect is essentially a dome of heat that surrounds densely-populated cities that tend to have numerous buildings, scarce greenery, a lack of open space, and lots of emissions and dark concrete.
That makes the record heat that has been suffocating cities this summer substantially worse.
"So in the future, things will only get worse," Rotta Loria said.
Li Cohen is a social media producer and trending content writer for CBS News.