Out in the brackish waters of the Baltic Sea lies the tiny island of Stenskär – Stone Island – named for the granite boulder on its summit, left behind by a departing glacier in the last ice age. The island is kidney-shaped and it’s a 10-minute walk from tip to tail. One sunny evening in early August, Stig Jansson is rinsing fishing nets at his boathouse on the island, while his son-in-law Jarmo Ylitelo cuts perch fillets to be smoked and sold. The pink granite of Stenskär, its pine forest and encompassing sea have been Stig’s home for 81 years. Just beyond the boathouse, his garden is radiant in the summer sunshine. Flowers are blooming in a German naval mine that Stig’s father hauled from the sea, made safe and recycled as a planter. ‘It’s being used for a more peaceful purpose than it was intended for,’ says Stig.

Stig and Jarmo fish most mornings when the weather is compliant. ‘I was supposed to retire 20 years ago, but somehow it never happened,’ Stig says as he pilots the boat out to the nets and watches Jarmo reel them in. ‘The younger generation needs a hand.’ Together, Stig and Jarmo represent the twin identities of this region. Both are Finns, but they have different mother tongues: Stig’s is Swedish, Jarmo’s Finnish.

Stenskär is part of the island chain that fans out from Finland’s southern city Turku like a handful of gravel thrown into the Baltic. Its 20,000 or so islands make it one of the largest archipelagos in the world, encompassing every size and shape, from grynnor – tiny bald outcrops – to öar – islands with woods and lakes that are big enough to support agricultural communities and small towns.

Geographically and politically, the archipelago is part of Finland, but the most widely spoken language is Swedish – a legacy of the region’s many centuries under Swedish rule. The archipelago is a place of deep silences and enchantment; even in summer, when its population is swollen by visitors who have come to unwind in their summer houses and stock up on sunlight for the winter ahead.

It’s a 90-minute drive from the airport at Turku to the island of Nagu, one of the gateways to the archipelago. From here, free public ferries head out to the surrounding islands.

On a sunny morning, Nagu’s harbour is as close as it ever gets to bustling. A few dozen shoppers are browsing in the boutiques along the wharf. A two-masted yacht cuts across the bay under full sail. The mouth-watering smell of freshly baked cinnamon buns rises from a bakery. Five minutes outside town, I find myself cycling a rented bike along a deserted road. Wind rustles the rye fields. There are a few feathers of high white cloud in an otherwise blue sky. I’m overwhelmed by the silence and space. The roadside is carpeted with bilberries, lingonberries, wild strawberries and, if you look carefully, caches of chanterelles.

Margot Wikström farms four hectares of berries at Tackork Farm in the island’s backcountry. She offers me a gooseberry. ‘You don’t have these in England. You can never have these in England!’ She laughs, aware how boastful she sounds. ‘It’s the light! The sweet taste comes from the the light. It gives them what we call Arctic aromas.’ The round-the-clock light of the northern summer has charged up the fruit with an unfamiliar sweetness and intensity.

Margot lives alone in the huge farmhouse, growing berries in the summer, managing her forest in the winter. She’s succeeded in taking up farming from scratch. She came here 10 years ago, quitting her job in a bank to start a new life as a farmer. ‘People told me, “You’ll never do it, you cannot do it.” They all sat in a bar in Nagu town telling each other I couldn’t do it, because I am a woman, because I worked in a bank. They didn’t know what kind of person I am on the inside.’

She might be atypical of the region’s farmers, but Margot follows in a long tradition of island resourcefulness and formidable Finnish women. Finland was the first country in the world to grant unrestricted voting rights to women in 1906, and elected its first female president in 2000.

To the lighthouse
In summer, the waterways between the islands teem with life. There is every kind of watercraft: passenger ferries, car ferries, sailing yachts, rowing skiffs, motorboats. Most thrilling of all are the rigid inflatable boats that whizz between the islands, cutting hour-long journeys to a matter of minutes.

Standing on an inflatable, holding on tightly, I feel like I’m watching the journey on fast forward. The islands rip past. The scene judders as the boat bounces over a series of waves. The tower of Bengtskär lighthouse appears in the distance, like a factory chimney upon an L-shaped chunk of granite. The building is faced with granite quarried from the island and, in bright sunshine, it presents a slightly fearsome aspect: half Colditz, half Hogwarts. It was built in 1906 to make this route secure for ships. West of the lighthouse lies a treacherous array of rocks and shallows.

At the top of the tower, two automated lamps do the work that once required the presence of five lighthouse keepers and their families. Their rooms are now let to the visitors who have made the 90-minute boat trip from the ports of Kasnäs and Hanko.

This place has seen its fair share of strife. It was bombed in WWI and attacked by the Soviet army in WWII. A plaque in the whitewashed chapel on the second floor commemorates those Finnish soldiers who died defending it.

Aside from the lighthouse and a few outbuildings, there is virtually nothing here except bare rock and a few wildflowers – knapweed, wild pansies – that have somehow found enough soil to grow in. But, this being Finland, one of the buildings houses a sauna, fuelled by wood that is brought to the island by boat. Finns are almost fanatical about the benefits of the sauna – which is in fact a Finnish word. In 1999, some of the country’s hard-core sauna enthusiasts instigated an annual world championships. Participants competed to see who could stay the longest in a sauna heated to 110˚C. The event was abandoned in 2010 after a Russian competitor died and the Finnish champon was hospitalised.

But, approached a little more sensibly, the sauna is a deeply pleasant and therapeutic experience. I step inside the tiny granite building. A couple of ladlefuls of water on the rocks above the stove raises the temperature to scalding. After about 10 minutes I emerge, sweaty and disoriented, and clamber down the rocks to the sea, expecting the cold to be shocking.

In fact, it’s unexpectedly lovely. The water is not salty at all – a quirk of this part of the Baltic – and its relative shallowness means that it warms up quickly in summer. Even at this northerly latitude, and miles from the mainland, the water is pleasantly swimmable. That night, in a simple whitewashed bedroom, I fall asleep immediately, but my dreams are full of water and I wake up rather confused, finding myself on a lone rock in the middle of the dark sea.

Art meets nature
Their tiny populations and the seemingly endless summer light make the islands places of serenity and contemplation.

Pia Rousku is an artist who lives on Korpo, an island next to Nagu. She curates an annual outdoor exhibition of art in the woods near her home. The pieces are shown along a path that is supposed to be walked without shoes. ‘I wanted people to feel nature under their feet,’ she says. ‘It’s something they’re not used to.’

Being barefoot does somehow make you more vulnerable and receptive to the art. The moss feels spongy and cool underfoot. Discreet signs shepherd you between a dozen artworks which you come upon unexpectedly. Arja Maarit Puhakka’s Once Upon a Time is a doorway made out of columns of unbound books. I think it must be about the power of books to transport the reader, but it also alludes to the paper’s previous life in trees.

Another of her installations illustrates the geological history of the island. Blue lines around tree trunks indicate the sea level 2,000 years ago. A small cairn of white stones shows all that would have been visible above the waves.

The archipelago is still rising from the sea – a slow-motion reaction to being compressed by glaciers for millennia in the last ice age. In Swedish, there is a dizzying number of synonyms for the word ‘island’. A helpful local jots down a partial list for me – ö, holme, kobbe, skär, grynna – and quickly comes up with 14 different words. Whether or not this is the biggest archipelago in the world depends on whether you count each grynna or sunken rock as a separate island. But what is undeniable is that there is more than a lifetime’s worth of islands to explore here.

‘It took me a long time to realise how special this place is,’ says Kaj Arnö, as he paddles a kayak across the bay of Pensar Syd, in the south of Pensar, an island six miles from Nagu. Kaj is an internet entrepreneur who spent much of his childhood here. He’s also a kayak enthusiast who organises events around the archipelago.

He leads a group of visitors out from the beach at Pensar Syd in rented kayaks. The sea water is dark jade in the sunshine and turns navy blue when the sun goes behind a cloud. After an hour or so of steady paddling, we reach an uninhabited island called Duvholm and eat a picnic on its sandy beach: salmon from the smokehouse on Nagu, pickled herring and the distinctive island bread – a soft, dark rye sweetened with malt.

The short distances between many of the islands make it perfect for this kind of day trip. But, on the way back, the wind has freshened and the swell is bigger. I’m a fairly confident kayaker, but my boat suddenly seems unstable in the water. I hurry ahead to get to the lee of Pensar, then realise I’m all alone. I paddle back 500 metres. Two of the kayaks have capsized. Luckily, the water is warm and, with some assistance, both of their occupants are able to get back in. Later, back in Pensar, sitting around a table piled with crayfish and with their mobile phones drying out in bags of rice, they laugh about their mishaps. But it’s a reminder that the sea can’t be underestimated.

Reminders of an unusual past
Each day in summer, a car ferry called Östern crosses back and forth between Nagu and Hanka, a small ferry stop just across the channel. Transport links are so vital that the ferries are considered to be extensions of the road network, hence why many of them are free.

On its way to Hanka, the boat puts in at Själö, a wooded island only a few miles wide, with no cars. The air of timeless peacefulness on Själö is deceptive. Like much of the archipelago, Själö has risen very recently, in geological time, from the floor of the Baltic. Parts of the archipelago are rising as quickly as one centimetre a year. Older residents are able to point out patches of dry land that didn’t exist in their youth. At the end of a winding path, a timber church appears in a clearing. It’s been re-roofed with scalloped wooden tiles and copper flashing. It was first built in 1733 for the residents of this island.

In its cool, rough-hewn pine interior, you get your first sense that something here was a little out of the ordinary. The western end of the church is separated from the rest of it with a wooden screen and the occupants of these pews had their own entrance.

From 1619, Själö housed a leper colony. They worshipped in this church, quarantined from the rest of the congregation. When the last leper died in 1785, the island was given over to a mental hospital, which finally closed in 1962.

Outside the church, the sunshine dispels any lingering disquiet. Boats of curious visitors dock at the marina at one end of the island, where Keijo Alastalo serves them coffee. He tells me that his family has been on Själö for three generations. His father was a nurse at the mental asylum. Keijo farmed as a boy, driving a pony in the hayfields that have now virtually vanished into the encroaching forests.

He has returned to spend his retirement on the island that he loves. I ask him what the mental hospital was like. ‘Not as bad as people say,’ he says. ‘The patients were free to wander the island. In the summer they’d pick berries and mushrooms. I remember working in the fields with one lady who didn’t seem mad at all. I said to her, “You seem normal. You could leave.” She looked around to make sure no-one was listening and said, “Keijo, I am not that crazy”.’

The article 'A portrait of summer in the Turku archipelago' was published in partnership with Lonely Planet Traveller.