Sydney is in a coastal basin bordered by the Pacific Ocean to the east, the Blue Mountains to the west, the Hawkesbury River to the north and the Woronora Plateau to the south. Sydney lies on a submergent coastline, where the ocean level has risen to flood deep river valleys (rias) carved in the hawkesbury sandstone. One of these drowned valleys, Port Jackson, better known as Sydney Harbour, is the largest natural harbour in the world. There are more than 70 harbour and ocean beaches, including the famous Bondi Beach, in the urban area.
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