Across the glittering waters of Sydney, Neil Oliver explores the network of fortification to protect 'Fortress Sydney', and discovers how close the city came to being taken in the Second World War. Xanthe Mallett discovers some ingenious colonial DIY, as she learns how to make building mortar out of oyster shells. Tim Flannery reveals the geological secrets of the city's vast and sprawling harbour, and unlocks the riddle of the rivers that had Captain Phillip baffled in 1788. Brendan Moar examines the Sydney Harbour Bridge, an international icon in engineering and design, while Emma Johnston hunts for tropical fish in Sydney's temperate and diverse harbour.