On the coast of modern Northern Ostrobothnia, the topography remains flat, but instead of the fields of modern Central and Southern Ostrobothnia, the land is largely covered with marsh. Inland the landscape is dominated by coniferous forests and hills, with an occasional village or small town. The most important geographical feature of inland Ostrobothnia is the 887 km2 lake Oulujärvi. Most activity in the region of Kainuu is centered around this lake, which is connected to the sea via the Oulujoki. North of Kainuu, Koillismaa forms one of the most rural and poorest sub-regions in Finland.