Legend holds that Pythagoras withdrew into a cave to study and reflect from within the Earth itself. Ghisolfi portrays the underground not as emptiness but as infrastructure in its most elemental form: a geological landform that provides stable temperatures, protection from the elements, and insulation from the lively, noisy surface. What appears here as a cave—a natural shelter—marks one step along a much longer human trajectory. For millennia before and after this scene, people have carved into the earth—hollowing out mines, chambers, and tunnels, and eventually constructing vast underground urban systems that extract resources, regulate environments, and sustain life above. Ghisolfi’s painting captures a moment in humanity’s long relationship with the underground, framing life below ground as part of a continuous progression from caves to cities underground.

For Dante, the underground is where excess, punishment, and permanence converge—a realm shaped by the weight of human choices. Beneath modern cities, a quieter inferno of our own making is taking form. Heat released by buildings and buried infrastructures seeps into the ground and accumulates over time, transforming the subsurface into a warming archive of urban activity. This underground inferno is reshaping the underworld as a deep signature of how we build and live.

Pen-and-ink drawing with collaged elements on paper. With its clear, cutaway view beneath a city, this illustration shows how buildings rely on the ground below for support. What appears solid and unchanging is, in reality, part of a reactive underground environment. As cities warm, heat released from buildings, tunnels, and buried infrastructure accumulates in the subsurface, gradually raising ground temperatures. This hidden warming can cause the ground to shift, construction materials to age faster, and foundations to deform over time. Macaulay’s drawing invites us to look beneath our feet and recognize, especially nowadays, that civil infrastructure is deeply connected to underground climate change—even when its effects remain out of sight. Image © David Macaulay.

Juxtaposed by more than two decades, these images reflect a profound transformation in the thermal experience of underground urban transit—from a system once perceived as merely congested and uncomfortable to one increasingly characterized by chronic and extreme heat. The recent depiction captures the intensification of thermal stress driven by rising surface temperatures, growing anthropogenic heat inputs, aging infrastructure, limited ventilation, and the cumulative effects of underground climate change. Together, these covers serve as a visual chronicle of how subterranean transit environments have evolved into critical hotspots of urban heat exposure, with direct implications for passenger health, worker safety, infrastructure durability, and the resilience of cities in a warming climate. Images courtesy of The New Yorker, © Condé Nast.

In Inferno, Canto XIII, Dante describes a pathless forest of gnarled, thorny trees—each one a human soul condemned to vegetate eternally—whose bark bleeds when broken, revealing hidden suffering beneath an apparently inert surface. This imagery offers a powerful metaphor for life underground subjected to extreme heat: soils, roots, fungi, microbes, and burrowing animals may seem stable but can undergo profound stress, disrupted natural processes, and irreversible damage when temperatures rise too high. Like Doréʼs forest, the underground can “bleed” silently, revealing harm only when disturbed or pushed beyond its limits. Image from Alamy/Premiergraphics.

In Shakespeareʼs tragedy, the famous soliloquy poses a fundamental dilemma: whether to endure the burdens of the present or to take action and transform oneʼs fate. This existential hesitation offers a compelling metaphor for the question confronting cities worldwide as they face underground climate change. Should existing buildings be retrofitted to reduce the heat they leak into the ground, or should urban development shift toward constructing new, highly efficient buildings? Much like Hamletʼs reflection, the answer lies not in a simple opposition but in a careful weighing of possibilities and consequences, to be assessed on a case-by-case basis. The real question is when to opt for one approach or the other, with the understanding that both are essential for the sustainable conservation and development of cities. Artwork information: Plexiglas polyhedron and sheets, reproductions printed on Plexiglas, crystal sphere (75 × 210 × 210 cm); Collection of the artist; © Giulio Paolini; Photo © Luciano Romano. Courtesy of Fondazione Giulio e Anna Paolini, Turin.

These posters illustrate a fundamental property of the subsurface: beyond a few meters, temperatures stay much more stable than in the atmosphere above, providing natural cooling in summer and warmth in winter. In modern cities, however, persistent waste heat from buildings and infrastructure often raises ground temperatures. The ground beneath cities, therefore, contains not only geothermal heat but also large amounts of waste heat. What once served as a natural thermal refuge has increasingly become a warming underground reservoir, highlighting both the hidden effects of urbanization and the opportunity to use this heat for sustainable heating and cooling in cities. Images © TfL from the London Transport Museum collection.

This artwork evokes a subtle yet powerful idea: light does not only arrive from above but can also emerge from beneath, passing through folded and colored layers to reveal depth, contrast, and hidden structure. What appears opaque becomes luminous, and what lies below begins to shape what is visible at the surface. Similarly, we now have the opportunity to bring visibility to underground climate change—shedding light on the concealed thermal impacts of urbanization while revealing the potential to harness this energy for more sustainable cities. Image courtesy of Archivio Claudio Rotta Loria; photo by Nicola DellʼAquila.

This visionary concept reimagines the underground as a central component of urban ecosystems, activating subsurface space even in emblematic contexts such as beneath the Arc de Triomphe. The underground has long played a fundamental role in human life—far more than we often recognize—and will become increasingly important in the future. The challenge lies in learning how to engage with this hidden domain thoughtfully: harnessing its potential while minimizing impacts and transforming it into a space that supports both human needs and environmental conservation. Image © Dominique Perrault Architecte, ADAGP.