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The streets, sidewalks and roofs of cities all absorb heat during the day, making some urban areas across the United States up to 6 degrees Fahrenheit hotter than rural ones during the day—and 22 degrees F hotter at night. These “urban heat islands”   can also develop underground as the city heat spreads downward, beneath the surface. And basements, subway tunnels and other underground infrastructure also constantly bleed heat into the surrounding earth, creating hotspots. Now the underground heat is building up as the planet warms.

According to a new study of downtown Chicago, underground hotspots may threaten the very same structures that emit the heat in the first place. Such temperature changes make the ground around them expand and contract (收缩) enough to cause potential damage. “Without anyone realizing it, the city of Chicago’s downtown was deforming,” says the study’s author Alessandro F. Rotta Loria, a civil and environmental engineer at Northwestern University.

The findings, published in Communications Engineering, expose a “silent hazard (危险)” to civil infrastructure in cities with soft er ground — especially those near water — Rotta Loria says. “There might have been structural issues caused by this underground climate change that happened, and we didn’t even realize,” he adds. While not an immediate or direct danger to human lives, this previously unknown effect highlights the impacts of a lesser-known component of climate change.

Similar to climate change above the surface, these underground changes occur over long periods of time. “These effects took decades, a century, to develop,” Rotta Loria says, adding that elevated underground temperatures would likewise take a long time to dissipate (逐渐消失) on their own.

But other researchers interviewed for this story all say this wasted energy could also be recycled, presenting an opportunity to both cool the subsurface and save on energy costs. Subway tunnels and basements could be updated with technologies to recapture the heat. For example, water pipes could be installed to run through underground hotspots and pick up some of the heat energy.