National Geographic embarks on an expedition into the mysterious world of ancient American history to shed a different light on the history of the Americas. 　　History books traditionally depict the pre-Columbus Americas as a pristine wilderness where small native villages lived in harmony with nature. But scientific evidence tells a very different story: When Columbus stepped ashore in 1492, millions of people were already living there. America wasn't exactly a "New World," but a very old one whose inhabitants had built a vast infrastructure of cities, orchards, canals and causeways. But after Columbus set foot in the Americas, an endless wave of explorers, conquistadors and settlers arrived, and with each of their ships came a Noah's Ark of plants, animals—and disease. In the first 100 years of contact, entire civilizations were wiped out and the landscape was changed forever.