Of all the creatures of the deep blue, none is as captivating as the octopus. In , marine biologist David Scheel investigates four major mysteries about these elusive beings. How can we study an animal with perfect camouflage and secretive habitats? How does a soft and boneless creature defeat sharks and eels, while thriving as a predator of the most heavily armored animals in the sea? How do octopus bodies work? And how does a solitary animal form friendships, entice mates, and outwit rivals? Over the course of his twenty-five years studying octopuses, Scheel has witnessed a sea change in what we know and are able to discover about octopus physiology and behavior—even an octopus’s inner life. Here he explores amazing new scientific developments, weaving accounts of his own research, and surprising encounters, with stories and legends of Indigenous peoples that illuminate our relationship with these creatures across centuries. In doing so, he reveals a deep affinity between humans and even the most unusual and unique undersea dwellers. Octopuses are complex, emotional, and cognitive beings; even as Scheel unearths explanations for the key mysteries that have driven his work, he turns up many more things of wonder that lurk underneath. This is the story of what we have learned and what we are still learning about the natural history and wondrous lives of these animals with whom we share our blue planet.