Parallel universes, strange hidden worlds, where an alternative reality exists have haunted science fiction for decades. Parallel universes sound like a myth, but a surprising number of leading scientists believe they are real. National Geographic delves into the mysterious science of other universes and discovers that many of physics most cherished theories actually predict that there are other universes beside our own. But what are they like? Could we ever visit them? And if we did, who or what would we meet? The reality is weirder than any science fiction.