Nothing intrigues us like a secret. And nothing entertains us as much as a secret revealed. In this LIFE Book, the editors of such other classic volumes as The Greatest Unsolved Mysteries of All Time look energetically into the hidden recesses of the world's prominent - and sometimes obscure - secret societies. From Yale's Skull and Bones to the Catholic church's Opus Dei, there are many mysterious organizations worldwide about which we think know some things, but not all. Until now. LIFE opens the doors, and peers inside. Famous for its photography, LIFE publishes here pictures from behind closed doors. Accompanying these pictures, many of which were first taken exclusively for LIFE, is an authoritative text reflecting the quickened pulse of a good mystery; the writers of both Unsolved Mysteries and the bestselling The Most Notorious Crimes in American History are behind this book. In special sections, LIFE will look back and explore historic quests: Did the RoundTable Really exist? Who was behind Easter Island and what was that society's dark secret? What was the clandestine Nazi group that went in search of Atlantis? Novelists and Hollywood, from Dan Brown to Indiana Jones, have often walked this way, and for good reason: We love this stuff. Now LIFE brings forth its vibrant treatment. This is an always fun investigation of the hidden world of secret societies.