As cities across Europe face rising temperatures and more frequent heatwaves, local authorities are exploring innovative ways to adapt. While green roofs, tree-lined streets, and water-sensitive urban design are gaining traction, one powerful climate ally often remains overlooked – the underground.
Beneath our feet lies an untapped resource with immense potential to buffer the impacts of extreme heat. Urban soil plays a crucial role in natural water infiltration and temperature regulation, especially when renaturalised or integrated into green infrastructure strategies. But there’s more: underground spaces themselves—often abandoned, neglected, or forgotten – could be repurposed to provide thermal refuge in times of need.
In many European cities, disused basements, tunnels, bunkers, and vast networks of historical cavities built for mining, defense, or religious purposes lie dormant. These subterranean environments are naturally shielded from temperature extremes, thanks to the isothermal properties of the ground. When properly secured, ventilated, and adapted for public use, they can offer low-cost, energy-efficient “climate shelters”, providing respite during the hottest days.
Moreover, rethinking underground spaces contributes to better urban planning and safety. Currently, these areas are often outside the scope of strategic city development, making them vulnerable to neglect, illegal dumping, or unsafe use. Mapping, securing, and integrating them into urban resilience plans not only improves public safety but also opens the door to a sustainable, circular use of city infrastructure.
As highlighted in the Covenant of Mayors’ latest feature on heat adaptation, cooler cities are resilient cities. The next frontier of urban climate action might just lie below the surface – where natural stability meets human ingenuity.
To unlock this potential, however, cities must invest in a deeper understanding of their subsurface environment. Urban geology plays a vital role in identifying suitable areas, assessing structural and thermal properties, and ensuring the safety and feasibility of underground reuse. Without this knowledge, we risk missing a key opportunity to design climate-smart, livable cities from the ground up.
Here are some compelling examples of cities using underground spaces as climate refuges or for thermal regulation, all based on real-world projects:
- Chongqing, Nanjing & Hangzhou (China).
During intense heatwaves, cities like Chongqing have opened air-raid shelters, while others like Nanjing and Hangzhou allow public access to metro stations as cool refuges underground (tomorrow.city). - Seville (Spain).
Drawing on ancient techniques, Seville has experimented with underground qanat canals to cool public spaces, a method rooted in Middle Eastern tradition (calcalistech.com). - Paris (France).
The city is tripling its underground cooling network, creating subterranean “cool islands” that complement surface green areas and museums in reframing urban heat refuge (bbc.com). - Singapore.
Under its Underground Master Plan, Singapore has implemented deep caverns – for oil storage and utilities – and a chilled-water central cooling system in Marina Bay, reducing building CO₂ emissions by ~40 % (wired.com). - Helsinki (Finland).
As part of its “City of Caves” master plan, Helsinki uses subterranean spaces, including its famous Temppeliaukio Church and underground swimming halls, for stable, year-round thermal comfort (wired.com).
Francesco La Vigna, ISPRA, Chair of EGS Urban Geology Expert Group