Read a chart of the Finnish archipelago and it looks like chaos. Sail it and a logic emerges: the thousands of islands break the Baltic swell into calm lanes, and a vessel that knows the route can travel enormous distances in sheltered water. Kyrksundet is one of the good links in that chain. Hitis was a trading harbour during the Viking age, and archaeological finds from the islands reach back to the eighth century - the traffic here predates almost everything written down about Finland. A Viking-age crew and a modern yacht want exactly the same thing from this coast: somewhere the wind cannot reach them, close to deep water, with an easy exit in the morning.

The strait took its name from what stood beside it. From the 1200s onward a succession of wooden chapels, each with its burial ground, occupied Kyrkon - Church Island - at Kyrksundet. The last of them measured nine and a half by eight metres and served the community until 1637, when work began on a replacement nearby. The present church, known as Andreaskyrkan after the parish vicar Andreas Prytz, was finished in 1686 and stands today as the second-oldest wooden cross church in Finland. Consider what that continuity means in practice. In one small stretch of water, people have been anchoring for a millennium, burying their dead for eight centuries, and worshipping in the same wooden building for close to three hundred and forty years - through Swedish rule, Russian rule, independence and two wars.

Administratively, Hitis belonged to Kimito until it became a municipality in its own right in 1861. It covered about 110 square kilometres of land distributed across all those islands, which produced a population density that barely means anything: 811 people in 1963, 732 by 1968, spread over water. The community was overwhelmingly Swedish-speaking - 88.9 per cent in 1960 - as much of the Aboland archipelago still is. Fourteen villages made up the whole: Bergo, Biskopso, Bolax, Bole, Hitis, Holma, Hogsara, Kaxskala, Kasnas, Rosala, Stubbnas, Tunhamn, Vanoxa and Vano. On 1 January 1969 the municipality was merged into Dragsfjard, which in 2009 was itself folded into Kimitoon. Hitis had been independent for 108 years and part of Kimito's orbit for many centuries either side of that.

The war reached these islands directly. On 26 July 1941, during the Continuation War, Soviet forces landed on the skerry of Bengtskar to destroy its lighthouse, which the Finns were using as an observation post to direct fire on the Soviet base at Hanko. Ten patrol boats put troops ashore against a garrison of 41 men under Lieutenant Fred Luther; 83 Finnish reinforcements and the gunboats Uusimaa and Hameenmaa turned the assault. The cost was severe on both sides of a very small rock. Thirty-one Finnish soldiers were killed and forty-five wounded, and eleven more died in a Soviet air attack the following day; Soviet losses were roughly thirty-one dead, with twenty-four men taken prisoner, among them survivors of the patrol boat PK-306 sunk beneath them. A second action was fought at Morgonlandet within Hitis waters. The lighthouse still stands, and so does the arithmetic: more than seventy young men died over the course of a day and a night on an outcrop you can walk across in minutes.

Getting here still means a boat. The ferry runs from Kasnas out to Langnas harbour on Rosala's northern shore, and Rosala - 8.2 square kilometres, roughly 140 year-round residents in the villages of Rosala, Bole and Stubbnas - functions as the archipelago's centre of gravity. The Rosala Viking Centre puts reconstructed buildings and vessels of the period on the ground for visitors, built rather than excavated, but placed where the trade route genuinely ran. From there, summer boats carry people out to Bengtskar. In winter the population contracts to the people who actually live here; in summer the cottages fill and the sounds crowd with hulls, doing what hulls have done in Kyrksundet since the eighth century.