They are giants—stretching more than 300 feet above the ground, with hidden gardens and mysterious predators thriving within their canopy. National Geographic reveals the unexplored environment of the California redwoods in an epic, year-long exploration. Obsessive redwood climber Steve Sillett of Humboldt State University explores their massive crowns, discovering new record-breaking trees, while escaping falling branches and crashing trees in the process. Down below, National Geographic Explorer-in-Residence Mike Fay charts the redwood range to size up the past and future of these primeval trees threatened in 21st century California.