The streets, sidewalks and roofs of cities all absorb heat during the day, making some urban areas up to six 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 diffuses downward, beneath the surface. And basements, subway tunnels and other subterranean infrastructure also constantly bleed heat into the surrounding earth, creating hotspots. Now that 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 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 on July 11 in Communications Engineering, expose a “silent hazard” to civil infrastructure in cities with softer 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.

“For a lot of things in the subsurface, it’s kind of ‘out of sight, out of mind,’” says Grant Ferguson, an engineering geologist at the University of Saskatchewan, who was not involved in the new study. The underground world teems with life, however. It is home to animals that have adapted to subterranean living such as worms, snails, insects, crustaceans and salamanders. These creatures are used to “very static conditions,” says Peter Bayer, a geoscientist at the Martin Luther University Halle-Wittenberg in Germany, who was also not involved in the paper. Aboveground temperatures often swing wildly throughout the year, but the subsurface remains around the yearly average temperature, he explains. In Chicago, that’s about 52 degrees F.

The subsurface has “a memory that air temperatures don’t have,” Ferguson says. As these stable temperatures rise because of climate change and underground urban development, scientists such as Ferguson and Bayer are keeping tabs on the potential implications for underground ecosystems. For example, if groundwater gets too warm, it could kill or drive away animals, trigger chemical changes in the water and become a breeding ground for microbes.

But the question of how underground hotspots could affect urban infrastructure has gone largely unstudied. Because materials expand and contract with temperature change, Rotta Loria suspected that heat seeping from basements and tunnels could be contributing to wear and tear on various structures.

He collected three years of temperature data from more than 150 sensors installed in basements, train tunnels and parking garages underneath Chicago’s downtown Loop district. For comparison, sensors were also installed in the ground beneath Grant Park, which is located in the Loop, along the shore of Lake Michigan.

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