4. Harbour City - Sydney

Sydney is home to a fifth Australia's entire population and is trapped between the Blue Mountains and the ocean, sitting on the driest inhabited continent on Earth, and surrounding one the planet's largest natural harbours. This episode uncovers what is inside Sydney's buildings and beneath its streets that allow the city to function - from the concealed contours of the harbour that allow super-sized ships to sail into the heart of the city, to the underground caverns that store its water.