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	Hot Spots About 4,000 million years ago the earth's surface was covered with active volcanoes.   Previous: Back Book: Incredible But True Section: Incredible But True Chapter: Restless Earth