Columbus did NOT think the world was round. Also, it was not commonly believed that the earth was flat at that time. In his own words from his Narrative of the Third Journey 1498-1500
"I have always read that the world of land and sea is all spherical. All authorities. . . have constantly drawn and confirmed this picture...I have found such great irregularities that I have come to [the conclusion]. . . that it is not round as they describe it, but the shape of a pear, which is round everywhere except at the stalk, where it juts out a long way; or that it is like a round ball, on part of which is something like a woman's nipple... [The Garden of Eden] lies at the summit of what I have described as the stalk of a pear. . . As I have said, I do not believe that anyone can ascend to the top. I do believe, however, that. . . waters may flow from there. . ."
In other words, Columbus thought the world was shaped like a pear or breast with Eden on the stem or nipple with water flowing from it. Hmmm. Not what I was taught in school.