Söderfjärden is a plain 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. It is currently cultivated and is very clearly visible from air as a large, circular field.