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Söderfjärden is a plain and partially a polder in Ostrobothnia, western Finland, 10 kilometres south of the town of Vaasa. The plain is in an impact crater which was made at least some 640 million years old (Proterozoic). The crater's diameter is 6.6 km and its maximum depth is 300 metres. It is filled with Cambrian sandstones leaving only the outer rim visible. There is also a central uplift, which is buried. After rising from the sea due to post-glacial rebound, Söderfjärden was a wetland but was later drained with help of a pump station. It is currently cultivated and is clearly visible from air as a large circular field. This makes Söderfjärden unique among the other impact structures in Finland which are at least partially below sea level.