Underneath downtown Chicago’s densely packed Loop district, the ground temperature is often about 10 °C warmer than the soil in nearby Grant Park. A three-year scientific study published in Communications Engineering reveals that subsurface infrastructure—including basements, parking garages, and subway tunnels—is altering urban geology and creating underground heat islands.
Here is why that matters for the long-term stability of major metropolitan areas: cities around the world are facing a hidden thermal shift that they were never engineered to withstand.
To uncover this subterranean phenomenon, researchers from Northwestern University deployed more than 150 wireless temperature sensors. Led by Alessandro Rotta Loria, the engineering team buried monitors across building basements, transit tunnels, underground parking facilities, and subterranean roadways such as Lower Wacker Drive.
For comparison, the team planted control sensors in the soil of Grant Park, a green space stretching alongside Lake Michigan. The setup mirrored a basic scientific control experiment: a thermometer placed in open parkland versus one tucked deep inside the parking in the basement of a city.
The data collected over three years exposed a stark thermal divide. While the park soil maintained natural temperatures, the earth beneath the Loop registered an increase of about 10 °C. Inside the underground structures themselves, air can reach temperatures up to 25 °C higher than the natural ground.
But there is a catch regarding how that heat travels. The surplus energy does not stay confined to transit tunnels or parking decks. It radiates outward, warming the surrounding earth and altering the physical properties of the soil.
Urban centers generate immense quantities of waste heat through human activity, commercial operations, and subterranean transportation networks. Researchers describe this underground temperature accumulation as the subterranean equivalent of the surface urban heat island effect, especially fueled by human activity.
To understand how this thermal footprint evolved over time, Alessandro Rotta Loria developed a three-dimensional computer model tracking ground temperatures from 1951—the year Chicago’s Loop subway tunnels were completed—up to the present day. Because the simulated data closely matched field measurements, the team used the model to project temperature trends forward to 2051.
The projections indicate that subsurface temperatures are rising by approximately 0.25 °F (0.14 °C) annually. That pace translates to roughly 1.4 °C of additional warming over ten years, a steady climb that continuously warms the clay-rich sediment underlying the city.
Unlike bedrock, fine-grained clay soils can contract when heated. According to the study’s findings, this constant heat exposure subjects building foundations in the city center to slow, continuous, and often undesirable settlement.
Metropolitan infrastructure is quietly shifting as a result. Simulation data indicate that the ground can swell upward by up to 12 millimeters or settle downward by as much as 8 millimeters over a period that runs until 2051.
| Metric | Measured or Projected Value | Context |
|---|---|---|
| Temperature Differential | About 10 °C warmer | Loop soil compared to Grant Park soil |
| Subsurface Air Variance | Up to 25 °C higher | Inside underground structures vs. natural ground |
| Projected Annual Warming | ~0.25 °F (0.14 °C) per year | Computer model projections running through 2051 |
| Simulated Ground Movement | -8 mm to +12 mm | Estimated foundation swelling and settling by 2051 |
These millimeter-scale deformations do not point toward an immediate structural catastrophe, and Alessandro Rotta Loria notes that safety risks must be evaluated on a case-by-case basis. Yet, the physical movement affects the daily operations of foundations and transit lines that were not designed for such temperature variations.
Global Metropolises Facing the Same Thermal Creep
Chicago is far from alone in experiencing this phenomenon. Researchers point out that subsurface urban heat islands affect almost every densely populated metropolitan area on earth. Researchers cite London, New York, Paris, Milan, or Miami, and consider London as probably the most affected.
Addressing the issue requires a shift in how engineers approach subterranean design. The most direct mitigation strategy involves insulating underground transit systems, commercial basements, and utility networks to prevent waste heat from escaping into the surrounding earth.
Where insulation proves impractical, geothermal technologies offer a viable alternative to capture and repurpose that waste heat. What began as an underground temperature survey in Chicago now serves as a blueprint for how cities worldwide must monitor and adapt to the unseen consequences of urban expansion.
