Wikipedia
Söderfjärden is a plain and partially a polder in Ostrobothnia, western Finland, just 10 kilometres south of the town of Vaasa. The plain is located in an impact crater at least some 640 million years old ( Proterozoic). The crater has a diameter of 6.6 km and a maximum depth of 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 by a pump station. It is currently cultivated and is very 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.