Technically known as "Subsurface heat islands," underground climate change is the warming of the ground under our feet, caused by heat released by buildings and subterranean transportation such as subway systems.

"The denser the city, the more intense is underground climate change," said lead study author Alessandro Rotta Loria, an assistant professor of civil and environmental engineering at Northwestern University in Evanston, Illinois.

'A silent hazard' Studied for the past 25 years, underground climate change can cause issues such as groundwater contamination or problems with underground railways by making tracks prone to buckling or causing passengers to become ill due to excessive heat.

Underground climate change can affect systems and structures such as underground railways, making tracks prone to buckling or causing passengers to become ill due to excessive heat. Bruce Leighty/Alamy Stock Photo Underground climate change can affect systems and structures such as underground railways, making tracks prone to buckling or causing passengers to become ill due to excessive heat.

Bruce Leighty/Alamy Stock Photo "It's important to stress that underground climate change does not threaten the safety of people and does not threaten to collapse structures and buildings," Rotta Loria said. "We can take action by applying thermal insulation to underground building enclosures, to minimize the amount of waste heat that goes into the ground," he said.

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