Taking a deep dive in Helsinki’s subterranean city

This is no cosmic mirage, but Helsinki’s acclaimed underground swimming hall, officially unveiled in 1993. Known locally as Itäkeskus uimahalli, it has since blossomed into one of the city’s most cherished and frequented leisure facilities, attracting an impressive 400,000 visitors annually. Here, a diverse tapestry of local residents, eager tourists, and families converge, drawn by its unique ambiance and accessible amenities. This particular immersion was part of a more extensive exploration, a week-long journey into Helsinki’s sprawling subterranean metropolis, undertaken by the author as one of 12 research trips over three years. These expeditions aimed to dissect how cities globally are being re-envisioned and re-engineered to navigate an increasingly uncertain future, culminating in the book, How to Build a City.

While many global metropolises boast some form of underground infrastructure – from intricate subway systems to utility tunnels – Helsinki’s subterranean network stands apart. It is widely considered the only one to have been comprehensively master-planned and transformed with a primary focus on peacetime functionality, while simultaneously retaining a critical defense capability. The genesis of this unique urban layer dates back to the Second World War, when initial bomb shelters were hewn from the bedrock. As the city expanded and its population burgeoned, a more efficient and less disruptive solution became imperative for essential urban infrastructure, such as heating plants, telecommunications cables, and wastewater treatment facilities. Instead of scattering these vital systems across expensive surface land, continually disrupting streets and sacrificing precious green spaces with intrusive excavations, Helsinki opted for a pragmatic and visionary approach: tucking them away beneath ground level, by expanding and repurposing the existing wartime shelters.

This strategic foresight led to a pivotal moment in 2010 when the Helsinki City Council formally approved what is recognized as the world’s first officially master-planned underground city. This monumental undertaking encompasses hundreds of vast, human-made caves and tunnels, some delving as deep as 90 meters below the surface. The Itäkeskus pool, a testament to this progressive vision, actually predated the formal approval of this comprehensive space allocation plan by 17 years, showcasing an early commitment to subterranean development.

In times of peace, these subterranean spaces are vibrant, integral parts of daily urban life. They house bustling shopping malls, expansive playgrounds, an exhilarating go-karting track, and critical subway stations, all interconnected by an astonishing network of nearly 200 miles (293 km) of tunnels. This seamless integration of underground and overground life allows Helsinki to maximize surface space for parks, residential areas, and cultural institutions, while maintaining essential services discreetly below. However, the dual purpose of this underground city is never truly forgotten. In the event of a national emergency or war, each of these facilities can be swiftly converted into a fortified bomb shelter, forming a robust network capable of accommodating an astounding 900,000 Finns – a capacity that significantly exceeds Helsinki’s current population, providing sanctuary for residents from the broader capital region.

The Itäkeskus swimming hall alone exemplifies this remarkable adaptability. Within a mere 48 hours, its massive pools can be drained, and the entire facility reconfigured with decontamination showers, transforming it into a fully functional shelter for almost 4,000 people. For the vast majority of its operational life, however, there are no overt clues to its latent defense nature. Instead, visitors are met with the comforting, familiar hallmarks of a public swimming pool: the invigorating tang of chlorine, the ambient warmth that permeates the air, and the joyous echoes of recreational activity.

My arrival in Helsinki coincided with the spring solstice, though the ground was still dusted with lingering, greyish snow, making the heated carriages of the 25-minute Metro ride from the city centre particularly welcome. Alighting at Itäkeskus station, I passed by a school nestled amidst Soviet-era tower blocks, where silver birches, their branches still bare, trailed over parked cars. From the outside, the municipal glass building housing the swimming hall presented an unassuming, almost uninspiring façade. Yet, stepping inside revealed a world entirely transformed.

A broad, imposing staircase swept me steeply downwards, flanked by dramatically rocky, whitewashed walls that immediately hinted at the excavated nature of the space. After paying the modest entry fee (Helsinki’s four public swimming halls offer excellent value at €6 for adults and €3.30 for children, making them popular with families and budget-conscious travelers alike), I proceeded to the changing rooms. These facilities branched off from a central granite core that soared cathedral-tall, emphasizing the sheer scale of the excavation. Despite signs being primarily in Finnish, the intuitive layout made navigation easy for non-natives, and the staff’s excellent English provided a reassuring presence.

The colossal hangar-like space unfolded before me. On one side, men bravely grimaced as they lowered themselves into a triangle-shaped plunge pool of icy water, a traditional Finnish ritual. On the other, a vibrant, postmodern dreamscape unfurled: a massive pool lined with a striking mosaic of lavender, canary yellow, and burgundy tiles. Overhead, huge plastic triangles in cerulean and lilac were suspended, adding a playful, artistic dimension to the cavernous space. 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, yet powerful, sound of moving water.

Swimming beneath this ersatz moon was, without exaggeration, an otherworldly experience. The unique lighting and architectural design created an immersive environment, blurring the lines between the mundane and the fantastical. After 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, in favor of the hot tub, steam room, and two saunas – essential components of the Finnish public bathing experience. A graphic on the sauna door, depicting a swimming costume crossed out in red, served as a gentle reminder of the local custom. I gingerly removed mine, sliding into place on the bench, consciously shedding any lingering British prudishness. Inside, the light was so dim that other bodies were barely discernible, creating an atmosphere akin to a private confession booth, fostering a unique sense of intimacy and communal relaxation.

Around me, conversations bubbled in a harmonious mix of Finnish, American English, and German, weaving together the quotidian details of living. I joined in, asking a stranger for dinner recommendations and whether to brave the plunge pool – small, human interactions that, if one didn’t consciously dwell on it, could make one completely forget the true, underlying purpose of this extraordinary place.

This dual identity, this seamless blend of daily life and strategic preparedness, was a theme I revisited repeatedly over the subsequent days as I continued my exploration of Helsinki’s underground. I wandered through the captivating, experimental installations at Helsinki’s underground art museum, Amos Rex (entry from €20; children go free). Its distinctive antennae-like skylights protrude from the pavement on concrete stalks near the chic Kluuvi shopping district, acting as a playful invitation to the hidden world below. Equally striking was the exquisite Temppeliaukio Church, often called the "Rock Church," nestled in the Töölö neighbourhood (€8 entry; children go free). Half-buried within a giant mound of rock, its raw, unhewn stone walls, copper dome, and exceptional acoustics create a truly spiritual and architectural marvel, another powerful example of building within and from the earth.

These sites, each a testament to Helsinki’s innovative spirit, are all, at their core, also potential bomb shelters. They serve as compelling evidence that urban planning can be profoundly responsive, forward-thinking, and aesthetically magnificent, even when designed with a sober awareness of potential threats. Helsinki unequivocally demonstrates that it is possible to achieve both robust resilience and inspiring beauty, creating a city that is not only prepared for an uncertain future but also enriches the daily lives of its inhabitants with verve and grace. This profound integration of civil defense with vibrant public spaces offers invaluable lessons for urban planners worldwide, showcasing a model of adaptive urbanism that is both pragmatic and deeply human.

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.