In the Baltic Sea, a vast archipelago is still rising from the ocean floor

Countries & Cities · Published 15 October 2026 · Updated 25 October 2026 Swipe more like this →

In short: The Archipelago Sea is a mosaic of thousands of islands in southwestern Finland, shaped by ancient geological forces and post-glacial land uplift. While traditional island life faces modern economic pressures, the region remains a unique biosphere reserve where the landscape itself is constantly evolving, growing, and shifting beneath the surface.

The Archipelago Sea is a remarkable coastal environment in the Baltic Sea, characterized by a dense, patchy distribution of islands. While estimates of the total number of islands vary based on definition, the region contains approximately 6,500 islands larger than 0.5 hectares, with some counts suggesting up to 50,000 if the smallest rocks and skerries are included. This topography is defined by a bedrock of granite, gneiss, and schist—remnants of the ancient Svecofennides mountain chain—which forms a peneplain. The area is also marked by three crater-like formations, including the genuine impact crater of Lumparn in the Åland region.

A defining feature of the archipelago is post-glacial land uplift, a process that began after the last ice age. As the earth's crust rebounds, the land rises at a rate of four to ten millimetres per year. This ongoing geological movement slowly creates new skerries and islands while enlarging or merging existing ones. Because the bedrock is composed of exceptionally hard rock, erosion occurs at a significantly slower pace than this uplift, ensuring the archipelago remains a dynamic, ever-changing landscape.

The region is home to a biosphere reserve that includes the Archipelago Sea National Park, established to protect both the natural environment and traditional cultural landscapes. Historically, island societies have relied on fishing and agriculture, though these livelihoods have become less profitable over time, leading to a marked population decline since the early 20th century. Today, the reserve serves to safeguard traditional ways of using natural resources, such as grazing and hay-cutting, while promoting environmental research. The area is divided between the autonomous, demilitarized region of Åland and the region of Southwest Finland, with the islands serving as a unique intersection of history, ecology, and geography.

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