The moon is an impostor. Swimming backstroke beneath its surface, I look up at craggy hollows in close relief. White rock undulates in all directions, a forest of peaks rising next to a desert of craters. Every time I trick myself into thinking I’m somewhere celestial, the sound of children splashing brings me back to earth.
This is no lunar landscape but Helsinki’s underground swimming hall, excavated from granite bedrock and officially opened in 1993. These days, Itäkeskus uimahalli is one of the Finnish capital’s most visited leisure facilities, welcoming 400,000 people a year. Local people mingle with tourists, I will discover, during a week exploring Helsinki’s subterranean metropolis.
My visit was one of 12 research trips I made over three years to better understand how cities are being reimagined. It turned into a book, How to Build a City, for which I travelled to locations that are testing the frontiers of urban living in response to an increasingly uncertain world.
Many global cities feature a version below ground, but Helsinki’s is thought to be the only one transformed for peace. As urbanisation increased the population, a more efficient solution was needed for heating plants and telecoms cables. You could scatter them around in expensive pockets of land, digging up the streets and sacrificing parks. Or you could tuck them away beneath ground level, by adding to bomb shelters first carved out during the second world war.
And that’s what Helsinki did. By 2010, its city council approved the world’s first officially masterplanned underground city, home to hundreds of vast human-made caves and tunnels up to 90 metres deep. Itäkeskus pool was opened 17 years before this space allocation plan was approved.
In peacetime, the caves function as shopping malls, playgrounds, a go-karting track and subway stations, all connected by almost 200 miles (293km) of tunnels. In the event of war, however, each can be transformed into a bomb shelter, part of a network capable of accommodating 900,000 Finns.
Itäkeskus swimming hall alone could house almost 4,000 people. In less than 48 hours it can be drained and fitted with decontamination showers. For the rest of the time, there are no clues as to its defence nature. Only the comforting hallmarks of the public pool: the tang of chlorine in my nose and the warmth that cloaked my skin.
I arrived in Helsinki on the morning of the spring solstice, although the ground was still dusted with greying snow and I was grateful for the heated carriages on my 25-minute Metro ride from the city centre. At Itäkeskus station, past a school surrounded by Soviet-style tower blocks, silver birches trailed bare branches over cars. From the outside, a municipal glass building gave an uninspiring welcome, but inside, I found a different world entirely.
A broad staircase swept me down steeply, framed by rocky, whitewashed walls. After paying (entry to all four of Helsinki’s public swimming halls is €6 for adults and €3.30 for children, making it popular with families as well as budget-conscious travellers), I disrobed in changing rooms spun off from a cathedral-tall granite core. Signs were in Finnish, but the space was easy for non-natives to navigate. Staff spoke excellent English.
At one side of the colossal hangar, men grimaced as they lowered themselves into a triangle of freezing water. On the other, a postmodern dreamscape was revealed, a large pool lined in lavender, canary yellow and burgundy tiles, with huge plastic triangles in cerulean and lilac suspended overhead. The only windows were set into a lifeguard’s tower next to a waterfall that filled the air with the roar of water in motion.
Swimming under the ersatz moon was an otherworldly experience. After 50 lengths, I’d earned my reward. Included in the entry fee is access to a gym, which I did not visit, plus a hot tub, steam room and two saunas, which I did. A graphic on the door showed a swimming costume crossed out in red and I gingerly removed mine, sliding into place on the bench and reminding myself that prudishness is a uniquely British affliction. Inside, it was so dark I could barely make out other bodies. The low light created the effect of a confession booth.
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Around me, conversations bubbled in Finnish, American English and German. The quotidian details of living emerged. I joined in, asking a stranger for dinner recommendations and whether I should brave the plunge pool. If you don’t dwell on it, you could forget the true purpose of this place.
It was something I kept returning to over the following days as I wandered through experimental installations at Helsinki’s underground art museum Amos Rex (from €20; children go free), whose antennae-like skylights popped up on concrete stalks from the pavement near the chichi shopping district of Kluuvi. And in the Töölö neighbourhood’s exquisite Temppeliaukio church (€8; children go free), half-buried in a giant mound of rock.
All were examples of a supremely responsive city. All were bomb shelters, too. Evidence that you can design for the future with verve and beauty. Helsinki shows it is possible to do it all.
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.