There are many cities. Some lie beneath the sea... Fly across iridescent tropical reefs. Brush through a cloud of countless jellyfish. Visit an alien world where the closer you look, the more you see. We think of reefs as exotic, distant places with little connection to our everyday world. Yet each of them is a hotspot of biodiversity, as vital to life on earth as the rainforests. Just as the Amazon has its secrets, so too do the reefs.