The urban underground is warming up.
Some seven centuries ago, Dante Alighieri offered a grim portrayal of the subsurface in “The Divine Comedy”. He described it as the location of hell, marked at its entrance by the inscription: “Abandon all hope, you who enter here.”
Long before Dante, ancient civilizations also viewed the underground as a realm of despair. Yet, paradoxically, the subsurface has always supported human life. In prehistoric times, it provided shelter from external threats. Over centuries, it evolved into a critical resource, hosting the infrastructure needed to sustain rapidly growing cities.
Today, major cities worldwide are teeming with underground infrastructure. However, this infrastructure generates substantial heat, which has leaked into the ground over centuries, giving rise to subsurface urban heat islands––an underground climate change. This brings multiple issues. First, underground climate change causes ground deformations beneath cities, which have the potential to compromise the durability of civil infrastructure and lead to issues like cracking. Second, it poses a significant public health concern, as excessively high underground temperatures can lead to heat-induced illnesses among commuters and workers. Lastly, it creates a major environmental issue, as rising ground temperatures disrupt subsurface ecosystems and alter biogeochemical cycles. These pressing issues highlight the urgent need for cities to act swiftly to address this silent but significant hazard.
Fortunately, while underground climate change is a silent hazard, it also presents a powerful opportunity. On one hand, it encourages the renewal of the existing building stock and the construction of new buildings and infrastructures by incorporating architectural and engineering solutions to create more efficient and sustainable spaces. On the other hand, it presents an additional motivation for deploying shallow geothermal technologies, which can redirect the heat driving underground climate change from where it is unnecessary to where it is needed—buildings and infrastructure, both of which are major thermal energy consumers.
In 2018, we transformed multiple building basements across Lausanne, Switzerland, into sensory environments. The goal was to explore how much heat is wasted in the ground beneath the city. The results indicate that each year, an amount of thermal energy equivalent to the annual consumption of 7,000 buildings is wasted in the city’s subsurface.
In 2019, we equipped multiple building basements, underground parking garages, tunnels, and subterranean streets across Chicago with sensors. The goal was to understand how waste heat deforms the ground beneath the city and its foundations.The results indicate that the infrastructure supporting Chicago—like that of many other cities worldwide—is shifting due to underground climate change.