The moon is an impostor.

Suspended in the cool, chlorinated embrace of the water, gazing upward, the illusion was almost perfect. Craggy hollows, sculpted and illuminated, stretched across my vision in close relief, mimicking a celestial body seen through the lens of a telescope. White rock, expertly carved from the Earth’s ancient crust, undulated in all directions, creating a surreal topography – a fantastical forest of peaks rising next to a barren desert of craters. It was a landscape that consistently tricked the mind, momentarily transporting me to some distant, ethereal realm. Yet, the persistent, joyful clamor of children splashing and laughing, their youthful exuberance echoing off the cavernous walls, served as an insistent tether, pulling me back from the imagined cosmos to the very real, very human experience on Earth.

This breathtaking subterranean vista was not, in fact, a lunar landscape, but the extraordinary Itäkeskus swimming hall in Helsinki, Finland. A marvel of engineering and design, it was meticulously excavated from solid granite bedrock and officially opened to the public in 1993. More than just a leisure facility, Itäkeskus uimahalli has since become one of the Finnish capital’s most iconic and frequently visited public amenities. Annually, it welcomes an astonishing 400,000 people, a testament to its popularity and accessibility. Within its unique environment, local residents seeking respite or exercise mingle effortlessly with curious tourists, all drawn to its distinctive charm. This single visit was merely the genesis of a week-long immersion, during which I would discover the astonishing depth and ingenuity of Helsinki’s subterranean metropolis.

My exploration of Helsinki’s hidden depths was a pivotal moment in a larger, three-year research endeavor, encompassing 12 distinct trips around the globe. This extensive journey aimed to better understand how contemporary cities are innovatively reimagining their structures and functions. In an increasingly uncertain world, marked by rapid urbanization, climate change pressures, and geopolitical instability, urban centers are compelled to test the frontiers of urban living. The insights gleaned from these travels culminated in my book, "How to Build a City: Journeys into the Urban Future," which delves into these groundbreaking urban experiments. Helsinki stands out as a prime example of a city proactively designing for resilience, efficiency, and quality of life, all while subtly preparing for potential future crises.

While many global cities possess extensive underground infrastructure – from the bustling metro systems of Tokyo and London to the intricate utility tunnels beneath New York, or even the historical catacombs of Paris and Rome – Helsinki’s subterranean network is widely regarded as unique. Its distinction lies in being the only such expansive system that has been explicitly transformed and master-planned for peacetime civilian use, while simultaneously retaining a critical dual-purpose function. The genesis of this remarkable underground city dates back to the Second World War, when initial bomb shelters were carved into the granite. Finland, sharing a long and often turbulent border with Russia, has historically prioritized civil defense, a legacy that profoundly shaped its urban planning. As Helsinki’s population rapidly expanded due driven by increased urbanization in the latter half of the 20th century, the city faced growing challenges in accommodating essential infrastructure. A more efficient and sustainable solution was desperately needed for critical services such as heating plants, telecommunications cables, and water networks. The traditional approach of scattering these facilities across expensive pockets of land, requiring disruptive street excavations and sacrificing precious green spaces, was deemed unsustainable. Instead, Helsinki opted for a visionary strategy: tucking these vital components away beneath ground level, strategically expanding and integrating them with the existing wartime shelters. This approach not only preserved surface aesthetics and functionality but also offered significant cost efficiencies and enhanced security.

This strategic decision marked a turning point for Helsinki. By 2010, after decades of incremental development and meticulous planning, the city council officially approved the world’s first master-planned underground city. This comprehensive blueprint laid out the future development and utilization of hundreds of vast, human-made caves and tunnels, some plunging up to an impressive 90 meters deep beneath the bustling city streets. The foresight displayed in the construction of the Itäkeskus swimming hall, opened a full 17 years before this comprehensive space allocation plan was formally adopted, underscores Helsinki’s long-standing commitment to subterranean development and its strategic vision for urban resilience. The city’s planners understood that the bedrock, while challenging to excavate, offered unparalleled stability, protection, and a readily available canvas for future expansion.

In times of peace, these subterranean "caves" and networks are vibrant, integral parts of daily urban life. They function as an astonishing array of facilities: bustling shopping malls, innovative playgrounds, a thrilling go-karting track, and efficient subway stations, all seamlessly connected by an intricate web of almost 200 miles (approximately 293 kilometers) of tunnels. This extensive underground realm also houses crucial utility networks, data centers, car parks, and sports facilities, all protected from the harsh Finnish winters and optimizing surface land use. In the event of war or a major disaster, however, each of these seemingly ordinary spaces can be rapidly transformed. They are part of a meticulously designed and interconnected network of bomb shelters, capable of accommodating a staggering 900,000 Finns – a figure that comfortably exceeds the city’s current population, ensuring readiness for residents and potentially those from surrounding areas. This dual functionality is not merely theoretical; it is a fundamental principle of Finnish civil defense, ingrained in the very fabric of its urban planning.

The Itäkeskus swimming hall exemplifies this ingenious dual-purpose design. Alone, this facility could house almost 4,000 people in an emergency. Its transformation from a public pool to a fully functional shelter is a testament to meticulous engineering and planning: in less than 48 hours, the massive pools can be completely drained, and the space can be rapidly fitted with essential decontamination showers and provisions for long-term habitation. For the majority of its existence, however, there are no overt clues to its latent defense capabilities. The experience for visitors is one of pure leisure, marked only by the comforting, familiar hallmarks of a public swimming pool: the invigorating tang of chlorine that greets the nose and the enveloping warmth that cloaks the skin upon entry, offering a stark and beautiful contrast to its hidden strategic purpose.

My arrival in Helsinki coincided with the morning of the spring solstice, a symbolic turning point, yet the ground remained stubbornly dusted with greying snow, a reminder of the lingering Nordic winter. I was profoundly grateful for the warmth radiating within the carriages of my 25-minute Metro ride from the city center, a journey that offered a glimpse into Helsinki’s efficient public transport system. Alighting at Itäkeskus station, I navigated past a local school framed by the stark, yet strangely familiar, lines of Soviet-style tower blocks, where silver birches, still bare, trailed their delicate branches over parked cars. From the outside, the municipal glass building housing the swimming hall offered an uninspiring, functional welcome. But stepping through its doors, I discovered a different world entirely.

A broad, inviting staircase swept me down steeply into the earth, its descent framed by rugged, whitewashed granite walls that immediately conveyed a sense of being within the bedrock itself. After paying the modest entry fee – a mere €6 for adults and €3.30 for children, making all four of Helsinki’s public swimming halls exceptionally popular with families and budget-conscious travelers alike – I proceeded to the changing rooms. These facilities were ingeniously spun off from a central, cathedral-tall granite core, an architectural feature that lent the entire space an unexpected grandeur. While the signs were predominantly in Finnish, the layout was intuitively designed, making it remarkably easy for non-natives to navigate. Furthermore, the attentive staff, proficient in excellent English, were always on hand to assist, ensuring a welcoming and stress-free experience for everyone.

Within the colossal hangar-like space, the main pool area unfolded. To one side, a smaller, triangular plunge pool drew a dedicated cadre of bathers, men grimacing good-naturedly as they bravely lowered themselves into its icy depths – a quintessential Finnish practice, echoing the traditional avanto or ice swimming experience. On the other side, a truly postmodern dreamscape was revealed: a vast main pool, its surface reflecting the vibrant hues of lavender, canary yellow, and burgundy tiles that lined its basin. Overhead, enormous plastic triangles in cerulean and lilac were suspended, casting playful, geometric shadows and adding to the surreal, otherworldly ambiance. The only hint of natural light, and indeed the only "windows," were subtly set into a lifeguard’s tower, positioned next to a cascading waterfall. This impressive water feature not only circulated the pool water but also filled the cavernous air with its constant, soothing roar, further enhancing the immersive, almost isolated sensation of being deep within the earth.

Swimming beneath this "ersatz moon" was, without exaggeration, an otherworldly experience. The artificial lighting, combined with the raw granite surroundings, created an illusion of being in a vast, alien grotto, far removed from the urban bustle above. After completing a satisfying 50 lengths, I felt I had earned my reward. The entry fee, remarkably, included access to not only a well-equipped gym (which I did not visit on this occasion) but also a blissful hot tub, a rejuvenating steam room, and two traditional Finnish saunas – all of which I eagerly explored. Approaching the sauna, a clear graphic on the door depicted a swimming costume crossed out in red, signaling the customary practice of nude bathing. With a slight gingerliness, a momentary flicker of my British cultural conditioning, I removed my costume and slid onto the wooden bench, reminding myself that such prudishness is indeed a uniquely British affliction, not shared in this relaxed Nordic environment. Inside, the profound darkness was striking; I could barely discern the other bodies present. This low, intimate light, coupled with the hushed atmosphere, created an unexpected effect, akin to stepping into a quiet confession booth, fostering a sense of shared, anonymous ritual.

Around me, the gentle hum of conversations bubbled in a medley of languages – Finnish, American English, and German – reflecting the diverse clientele. Amidst the steamy tranquility, the quotidian details of human living emerged naturally. I found myself joining in, asking a stranger for dinner recommendations for the evening and seeking advice on whether to brave the exhilarating cold plunge pool. It was a remarkable testament to the design and atmosphere of the place that, if one didn’t consciously dwell on it, it was entirely possible to forget the true, profound, dual purpose of this extraordinary facility. The seamless integration of leisure and readiness was a quiet, understated triumph.

This remarkable duality was a theme I revisited repeatedly over the following days as I continued my exploration of Helsinki’s subterranean marvels. My journey took me to the underground art museum, Amos Rex (entry from €20; children go free). Its avant-garde spirit is signaled by its distinctive antennae-like skylights, which emerge like futuristic concrete stalks from the pavement near the fashionable shopping district of Kluuvi, inviting curiosity. Inside, I wandered through experimental installations, experiencing art in a truly unique architectural setting, knowing that this modern cultural hub also served a deeper, protective function. Equally compelling was the visit to the Töölö neighbourhood’s exquisite Temppeliaukio Church (entry €8; children go free), famously known as the "Rock Church." This architectural masterpiece is literally half-buried within a giant mound of natural rock, its copper-domed interior bathed in natural light filtering through a skylight ring. Celebrated for its exceptional acoustics, it is a popular venue for concerts and events, yet, like its modern counterparts, it too is engineered to function as a bomb shelter, ready to protect in times of need.

Each of these sites – the aquatic wonderland of Itäkeskus, the artistic innovation of Amos Rex, and the serene beauty of Temppeliaukio Church – served as powerful examples of a city that is supremely responsive to the complex demands of the modern era. All were, at their core, meticulously designed bomb shelters, yet they also embodied verve, beauty, and functional elegance. They stood as irrefutable evidence that it is possible to design for the future, for resilience, and for the well-being of a populace, without sacrificing aesthetic appeal or daily utility. Helsinki, through its ingenious and integrated subterranean world, unequivocally demonstrates that it is indeed possible to do it all, setting a powerful precedent for urban planning worldwide.

How to Build a City: Journeys into the Urban Future by Gabriella Bennett is published by Headline (£25). To support the Guardian buy a copy at guardianbookshop.com. Delivery charges may apply.