Much of the Limestone Coast is low-lying, and was inundated by sea as recently as 2 million years ago. It had previously also been flooded 15-20 million years ago. The plains are lined by rows of low sandhills parallel to the coast, created at times when the coastline was at that level. Prior to European settlement, much of the land between the sandhills was swamp fed by streams and subject to inundation. A network of drains totalling 1450 km has been constructed to channel the water away through the sandhills to the ocean. Important areas of wetland remain including the lakes and lagoons such as the southern end of the Coorong and Bool Lagoon. Meanwhile, areas of upland in the Limestone Coast include the volcanic craters of Mount Gambier.