As Europe experiences another summer of exceptional heatwaves, cities across the continent are searching for urgent solutions to protect citizens from extreme temperatures. Recent records tracked by the Copernicus Climate Change Service highlight how severe these heat events have become, reaching during past June the warmest recorded values. While surface measures like green roofs, tree-lined streets, and urban parks are gaining ground, one powerful climate ally remains under our feet: the subterranean environment.

Nature’s underground thermostat

Beneath city streets lies an untapped resource with immense potential to cushion the impact of extreme heat. Urban soil plays a key role in natural water infiltration and temperature control, especially when integrated into green infrastructure.

What makes the subsurface particularly effective is its thermal inertia. With the exception of areas affected by specific thermal underground conditions (geothermal or permafrost areas) below a depth of approximately 3.5 metres, surface air temperatures generally no longer have an impact. At this depth and beyond, depending on geographical areas, ground temperatures can remain naturally constant at around 16-17 °C all year long.

Because of this natural thermal stability, subterranean spaces – many of which currently lie disused, neglected, or forgotten – can be repurposed to offer cooling during extreme weather. Across Europe, historical caves, disused basements, tunnels, and former bunkers built for defence, mining, or religious purposes offer environments shielded from intense heat. When properly secured, ventilated, and adapted to public use, they provide low-cost, energy-efficient “climate shelters” during the hottest days of the year.

European cities turn to the subsurface: lessons from Rome

Drawing inspiration from these principles, the city administration of Rome is reflecting on how the vast network of existing underground spaces beneath the Italian capital could be repurposed as climate shelters during future heatwaves, subject to the necessary safety and security evaluations and required works.

Integrating underground spaces into urban planning also improves municipal safety and sustainability. Subterranean areas are frequently excluded from city masterplans, leaving them vulnerable to neglect or unsafe use. By mapping, securing, and incorporating these spaces into climate adaptation strategies, cities can improve public safety while enabling a circular, sustainable reuse of urban infrastructure. Cooler cities are resilient cities.

The role of Urban Geology

Unlocking this subsurface potential requires a solid understanding of what lies beneath our feet. Urban geology plays a vital role in mapping suitable areas, evaluating structural and thermal properties, and guaranteeing the safety and feasibility of underground adaptation projects. Without geological knowledge, cities risk missing a key opportunity to build climate-resilient environments from the ground up.

See recent media coverage on this matter:

La Repubblica newspaper article (in Italian)
Roma cerca rifugio sottoterra: cave e gallerie antiaeree contro il caldo estremo

TG3 News Television reel (in Italian)
Watch here

El tiempo newspaper article (in Spanish)
Roma busca escapar del calor bajo tierra: la ciudad explora sus cuevas como refugios ante las temperaturas extremas