Peat is a bookkeeping error in the carbon cycle. Sphagnum moss grows, dies, and settles into water so acidic and so starved of oxygen that the bacteria which would normally finish the job simply cannot work. Each year's growth presses down on the last. Nothing is subtracted. Finland's bogs began forming after the last ice sheet withdrew from this landscape, and Torronsuo has been quietly adding to itself ever since, layer on waterlogged layer, until the accumulation lifted its own centre clear of the surrounding land. Twelve metres of peat is not a depth so much as an archive - every millimetre of it laid down by moss that was alive on a particular Finnish summer and never given the chance to disappear.

Roughly a hundred bird species nest on the bog, which for an open, treeless, apparently empty landscape is a remarkable figure. What makes the list unusual is not its length but its geography. Many of the birds and flying insects at Torronsuo belong to the north - species you would expect in Lapland rather than in Kanta-Häme, a couple of hours' drive from Helsinki. A raised bog is functionally a piece of subarctic Finland stranded in the south, cold, wet, nutrient-poor and open to the sky, and the animals adapted to those conditions found it and stayed. The park is valued for its butterflies for exactly the same reason. Torronsuo is a Ramsar site, listed under the international convention on wetlands of global importance.

You cross it on duckboards. Routes of 1.5, 8.5 and 10 kilometres run out from the Kiljamo parking area, and a separate 5.3-kilometre circuit loops the neighbouring Piippurinsuo bog. The planks are the only reason the interior is reachable at all, and they change how the place feels: the surface flexes under your weight, and the horizon is unbroken in every direction, which is a thing southern Finland almost never offers. At Kiljamo a 17-metre tower lifts you above the moss for the view that makes sense of the dome. A second tower at Idänpää was built in 1997 by volunteers on private ground. That same year a forest fire burned through nearby Tammela woodland, and a piece of the burnt ground was afterwards taken into the park - a rare case of a national park growing by accident.

Torronsuo was already a protected area before it became a national park in 1990, which matters more than the paperwork suggests. Across the twentieth century Finland drained an enormous share of its peatlands for forestry and agriculture, ditch by ditch, and a bog that survives in near-natural condition this far south is a survivor rather than a default. The park now draws more than fifty thousand visitors a year, most of them walking the boardwalks in the low golden light of late summer, when the cloudberries ripen and the moss turns a colour somewhere between rust and wine. Research published in 1996 described Torronsuo as Finland's largest raised bog massif - several bog units that have grown together into one continuous system. Standing on the tower at Kiljamo, that is exactly what it looks like: not a wetland with edges, but a country of its own.