Three of the most important words in science are "I don't know." Not knowing implies a Universe of opportunities--the possibility of discovery and surprise. Our understanding of science has advanced immeasurably over the last five hundred years, yet many fundamental mysteries of existence persist: How did our Universe begin? How big is the Universe? Is time travel possible? What's at the center of a black hole? How did life on Earth arise? Are we alone? What is consciousness, and can we create it? These mysteries define the scientific forefront--the threshold of the unknown. To explore that threshold is to gain a deeper understanding of just how far science has progressed. Covering time, space, matter, life, and consciousness, Krauss introduces readers to topics that will shape the state of science for the next century, providing us all passport to our own journeys of exploration and discovery.