The approach to the Port of Pori runs past a shoal called Trekanti, close enough to the fairway that a loaded timber ship feeling its way in at dusk had a real chance of finding it. A first beacon went up in 1851, but the Crimean War claimed it just three years later, and the shoal went unmarked again until a light shelter went up on the island of Kallo in 1884; that beacon was first lit on 15 September 1885. This was harbour lighting rather than landfall lighting - a short-range marker meant to define one channel, not to announce the coast of Finland to vessels sixty kilometres offshore. The distinction matters for understanding why Kallo looks the way it does. Everything about it is close-quarters work: modest height, a light showing white, red and green in fixed sectors so a helmsman could read his position in the channel by colour alone.

The structure standing today dates from 1903 and came from the drawing board of Gustaf Nyström, one of the leading architects working in the Grand Duchy of Finland at the time. That a designer of his standing was handed a navigation aid on a rock outside a timber port says something about how much the Port of Pori mattered in 1903. What he produced is an octagonal iron tower, painted white, with a wooden keeper's house beside it. The lantern sits about eleven metres above the sea and throws its isophase white-red-green signal on a two-second cycle out to roughly seventeen kilometres. For a lighthouse, that is a short reach. For the Mäntyluoto fairway, it is exactly enough.

In fog the light is useless, which is why Kallo's acoustic history is longer and stranger than its optical one. The first fog signal, installed in 1901, was a Hansen siren cranked by hand - somebody stood on the rock in the weather and turned a handle. In 1906 that was replaced by a mechanical arrangement of ball motors and a compressor producing the three-blast pattern that made the station locally notorious. When the light was converted to electricity in the 1930s, the compressed-air machinery gave way to an electric nautophone. Each upgrade made the horn easier to run and, in the way of these things, no quieter. Fog signalling has largely disappeared from modern navigation, displaced by radar and satellite positioning, but for half a century the sound of Kallo was how Pori knew the visibility had gone.

Kallo does not stand alone. The Mäntyluoto area around it is a nearly intact working maritime settlement of the late nineteenth and early twentieth centuries: the Pori pilot station, the coast guard station, a cafeteria, and the clubhouse of Segelföreningen i Björneborg, founded in Pori in 1856 and reckoned both the oldest yacht club and the oldest sports club in Finland. Finland's oldest harbour crane survives here too, along with the red-and-white wooden beacon of 1902-03 known locally as the Russian Tower, an Art Nouveau oddity in timber. The Finnish Heritage Agency has designated the whole ensemble a nationally important built cultural environment, protecting not one landmark but the relationship between all of them.

Most Finnish lighthouses require a boat, a permit, decent weather, and a tolerance for being stranded. Kallo requires a car. It is one of only two lighthouses in the country - the other being Marjaniemi on Hailuoto - that you can park directly beneath, and that single fact has made it one of the most photographed navigation aids in Finland. There is something honest about its popularity. A tower that spent a century keeping timber freighters off a shoal is now mostly encountered by people carrying a coffee out from the cafeteria to sit on the rocks and watch the pilot boats work the channel. The ships still come in past Trekanti. The light still marks the turn.