Taking a deep dive in Helsinki’s subterranean city

This extraordinary subterranean realm is not a distant lunar outpost, but rather Helsinki’s acclaimed underground swimming hall, the Itäkeskus uimahalli. Carved meticulously from the unyielding granite bedrock that underpins the Finnish capital, this architectural marvel officially opened its doors in 1993. Decades later, it stands as one of Helsinki’s most cherished and frequented leisure facilities, drawing an impressive 400,000 visitors annually. It is here, within these cavernous, rock-hewn walls, that local residents and international tourists alike converge, a melting pot of cultures seeking recreation and respite. This immersive encounter at Itäkeskus would serve as a pivotal introduction to a more extensive exploration of Helsinki’s remarkable subterranean metropolis, a journey I undertook over the course of a week.

My visit to Helsinki was one of twelve meticulously planned research expeditions conducted over three years, each designed to delve into the innovative ways cities worldwide are reimagining their urban fabric. These journeys culminated in my book, "How to Build a City," which chronicles pioneering approaches to urban living in an increasingly complex and unpredictable global landscape. While many major cities across the globe possess some form of underground infrastructure, Helsinki’s extensive sub-surface network distinguishes itself by being, uniquely, transformed and master-planned for peace, yet retaining an inherent capacity for defence.

The genesis of Helsinki’s underground city is rooted in a pragmatic response to escalating urbanisation and the inherent challenges of dense city living. As the population expanded, the city faced a critical need for more efficient and less disruptive solutions for essential services like heating plants, telecommunications cables, and waste management. The conventional approach—scattering these utilities across valuable surface land, necessitating disruptive street excavations, or sacrificing precious public parks—was deemed unsustainable. Helsinki, renowned for its forward-thinking urban planning, chose an alternative path: to tuck these vital infrastructures away beneath ground level, strategically expanding upon a network of bomb shelters originally hewn from the bedrock during the harrowing years of the Second World War. This decision was not merely about utility; it was a testament to Finnish resilience and foresight, a nation acutely aware of its geopolitical position.

This far-sighted strategy truly solidified in 2010 when Helsinki’s city council formally approved the world’s first officially master-planned underground city. This monumental undertaking encompasses hundreds of vast, human-made caves and an intricate labyrinth of tunnels, some plunging up to 90 metres deep. The Itäkeskus uimahalli, a testament to this vision, had already been operational for 17 years prior to the official approval of this comprehensive space allocation plan, showcasing the city’s early commitment to dual-purpose subterranean development.

In peacetime, these colossal underground spaces hum with life and activity, fulfilling a diverse array of civilian functions. They house bustling shopping malls, vibrant playgrounds where children’s laughter echoes off granite walls, a thrilling go-karting track, and critical subway stations, all interconnected by an astonishing network of nearly 200 miles (293km) of tunnels. This seamless integration of everyday urban life beneath the surface represents a triumph of engineering and urban design. However, the true genius of Helsinki’s underground city lies in its inherent adaptability. In the event of a national emergency or war, each of these seemingly innocuous facilities can be swiftly transformed into a robust bomb shelter, forming part of a sophisticated network capable of providing sanctuary for an astounding 900,000 Finns – a population roughly equivalent to the entire metropolitan area of Helsinki. This dual functionality underscores a profound commitment to national security and civilian protection, a legacy of Finland’s historical experiences and its ongoing strategic independence.

The Itäkeskus swimming hall itself embodies this dual-purpose design with remarkable efficiency. In its capacity as a civil defence shelter, it can accommodate almost 4,000 people. Should the need arise, the entire complex can be drained of its water and rapidly converted, fitted with essential decontamination showers and other emergency provisions, all within a remarkably swift 48-hour timeframe. Yet, for the vast majority of its existence, there are no overt clues to its latent defence capabilities. Instead, visitors are enveloped by the comforting, familiar hallmarks of a public swimming pool: the invigorating tang of chlorine that greets the nose and the pervasive warmth that cloaks the skin, creating an atmosphere of safety and normalcy.

My arrival in Helsinki coincided with the crisp morning of the spring solstice. Despite the calendar’s promise of renewal, the ground was still dusted with a persistent layer of greying snow, making the heated carriages of my 25-minute Metro ride from the city centre a welcome luxury. Alighting at Itäkeskus station, I passed a school nestled amidst Soviet-style tower blocks, where silver birches, still bare of their spring foliage, trailed skeletal branches over parked cars. From the outside, the municipal glass building housing the swimming hall presented an uninspiring, almost austere facade. Yet, stepping inside, I discovered a world utterly transformed.

A broad, inviting staircase swept me down steeply into the earth, flanked by raw, rocky walls that had been whitewashed, creating a striking contrast between natural geology and human design. After paying the modest entry fee—€6 for adults and €3.30 for children, making it an accessible and popular choice for families and budget-conscious travellers alike—I proceeded to the changing rooms. These facilities branched off from a central, cathedral-tall granite core, an awe-inspiring space that emphasised the sheer scale of the excavation. Despite the signs being predominantly in Finnish, the intuitive layout made navigation effortless for non-natives, and the staff, demonstrating typical Finnish hospitality, spoke excellent English, ready to assist.

Within the colossal hangar-like space, a vibrant tapestry of activity unfolded. To one side, men, with expressions of grim determination, slowly lowered themselves into a small, triangular plunge pool of icy water—a bracing Finnish tradition. On the other, a breathtaking postmodern dreamscape revealed itself: a sprawling main pool lined with an audacious mosaic of lavender, canary yellow, and deep burgundy tiles. Above, enormous plastic triangles in cerulean and lilac were suspended, casting playful reflections and adding a whimsical, almost futuristic touch. The absence of natural light was mitigated by expertly placed artificial illumination, and the only "windows" were cleverly integrated into a lifeguard’s tower, positioned next to a cascading waterfall whose constant roar filled the air with the soothing, powerful sound of moving water.

Swimming beneath this ersatz moon, with the overhead lighting mimicking the dappled glow of a celestial body, was an unequivocally otherworldly experience. Each length became a meditative journey through a carefully constructed illusion. After completing a satisfying 50 lengths, I felt I had earned my reward. Included in the entry fee was access to a well-equipped gym, which I bypassed, but also a luxurious hot tub, a purifying steam room, and two traditional saunas, which I eagerly embraced. A discreet graphic on the sauna door, depicting a swimming costume crossed out in red, served as a gentle reminder of Finnish sauna etiquette. With a slight hesitation, I gingerly removed mine, sliding onto the wooden bench and reminding myself that prudishness is often a uniquely British affliction. Inside, the dim light was so pervasive that I could barely discern other bodies, fostering an atmosphere of anonymous intimacy, akin to a confession booth where hushed conversations and quiet contemplation were the only requirements.

Around me, the air buzzed with a rich tapestry of languages – the soft cadences of Finnish, the confident tones of American English, and the distinct rhythm of German. It was here, in this shared space of vulnerability and relaxation, that the quotidian details of human connection emerged. I found myself engaging with a stranger, seeking recommendations for dinner and musing aloud about the wisdom of braving the plunge pool. In these moments, enveloped by warmth and camaraderie, it was remarkably easy to forget the profound, dual purpose of this extraordinary place.

This underlying theme of adaptability and hidden resilience was a concept I found myself continually revisiting over the following days as I continued my exploration of Helsinki’s underground wonders. I wandered through experimental installations at the city’s innovative underground art museum, Amos Rex, where its distinctive antennae-like skylights protrude from the pavement on concrete stalks, subtly punctuating the chichi shopping district of Kluuvi. (Entry from €20; children go free). And in the tranquil Töölö neighbourhood, I discovered the exquisite Temppeliaukio Church, famously known as the "Rock Church." Half-buried within a giant, naturally occurring mound of rock, its copper-domed interior is a masterpiece of light and sound, carved directly from the ancient stone. (Entry €8; children go free).

Each of these sites – the Itäkeskus uimahalli, Amos Rex, and Temppeliaukio Church – served as powerful examples of a supremely responsive city, one that embraces architectural ingenuity and innovative urban planning. Crucially, each also retained its inherent capacity as a bomb shelter, a silent testament to Helsinki’s integrated approach to civic and civil defence infrastructure. They stand as compelling evidence that it is entirely possible to design for an uncertain future with both audacious verve and profound beauty. Helsinki, through its visionary subterranean city, demonstrates unequivocally that it is possible to achieve both resilience and aesthetic excellence, creating urban spaces that are not only functional and secure but also enriching and inspiring.

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.