A look at how a dung beetle standing on its head can roll a ball in a straight line; if egrets ever regret hanging out next to hungry alligators; and what ghostly creature was caught on camera 3000 feet below the ocean's surface.
Going down into these narrow flooded passageways is not for the faint-hearted but, with only an estimated one percent of the caves explored, it is an opportunity for the team to write themselves into cave diving history, by pushing further in than anyone has before. This is a challenge that tests even the most accomplished cave divers. Steve must face the terrors of being lost in an underwater silt cloud in a cave. But there is a bigger issue at stake. All life in the Yucatan depends on the fresh water in this network of caves, but it's being contaminated by human development. With every metre the team maps, it adds to the knowledge of the system, which, in turns, helps protect it for future generations.
Part 3: Survival When David Attenborough first visited the Great Barrier Reef in 1957, he considered it the most spectacular place in the natural world and he assumed that it would last forever. Since then the coral has been dying at an unprecedented rate. In this episode he undertakes his most important mission to understand what the next few decades hold for this remarkable community of animals, as well as what is being done to save it. On his journey, David uncovers the remarkable ways in which scientists are trying to preserve the reef and embarks on an ambitious exploration of his own, the deepest dive ever on the reef. But despite the discoveries now being made by scientists and in the reef's furthest reaches, David fears for the future of this incredibly complex and beautiful ecosystem so vital for the world's oceans.