Overview: 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 WW2. Xanthe Mallett learns how to make lime as the convicts did in her quest to understand the importance of oysters in building the early colony. Tim Flannery reveals the geological secrets of the city's vast and sprawling harbour and unlocks the riddle of the rivers that had Captain Philip baffled in 1788. Brendan Moar examines an international icon in engineering and design, and reveals the story of those who made the greatest sacrifice. Emma Johnston hunts for tropical fish in Sydney's temperate and diverse harbour.