Many credit Columbus for being the first to discover America (cough, Leif Erikson), and many also credit Columbus for being the first person to think the world was round. But, all of those smart toga guys in Ancient Greece had known the Earth was round for well over 1000 years before him. In fact, they taught it in their schools. So, why wasn't this universal knowledge by 1492? What knowledge might people still ignore 1000 years from now that you can't believe isn't widely understood today?