Sometimes a valley has been formed in the hills near the coast of a country. Movement in the Earth's surface may cause the land to sink lower and become flooded by the sea. The shape of the valley can still be seen from the tops of the hills that stick out from the water. Some of the hills may become islands, and others become the shore of a bay. Sunken valleys often make good harbours. The east coast of Australia has many sunken valleys of which the most famous is Sydney Harbour. 