S1, E2: It hosts nearly 5 million tourists a year, but visitors see only a tiny fraction of the Grand Canyon's true beauty. Some of its backcountry has almost never felt the tread of a human foot. Even scientists know precious little about the canyon's 91 mammal species and more than 350 types of birds, its vegetation, and how and when it was formed. Now, National Geographic joins a rare scientific expedition to explore the canyon's entire 277-mile length. Explore the canyon's geological history, study the microscopic colonies of vegetation that survive where no other plant can and examine its many unique creatures to unlock some of the mysteries of this wonder of the world.