Ages 6-10. Did you know the center of the Earth was hollow? Do you know the words that can take you there, if you're under the covers in your swimming suit and prepared for the trip? Can you imagine traveling anywhere in the world where there's a swimming pool -- and being home again in time for breakfast? If you haven't been to Tube World yet, this book will tell you the secrets you need to know. And it will tell you about some children who discovered Tube World and used it to make the whole world a better place.