Incredible But True Incredible But True Human World History Makers Cities Of The World Art And Music Sporting Headlines Plant Life Fish, Amphibians, Reptiles Birds and Mammals Creepy Crawlies Marine Life Restless Earth Earth's Secrets Changing Weather Seas And Oceans Comparisons Amazing Structures Machines And Man World Of Inventions Travelling Around Mysterious Universe	Long History Universe It is difficult to imagine, but the universe is estimated to be between 10 and 18 billion years old.   Previous: Back Book: Incredible But True Section: Incredible But True Chapter: Mysterious Universe