Christopher Columbus and a Flat Earth

The ancient Greek Eratosthenes figured out the curvature of the Earth before the time of Christ

The common misconception that people before Columbus believed that Earth was can be traced back to a book by Washington Irving, published in 1828.

Even before Eratosthenes, it was common (but unproven) knowledge in the ancient world that the Earth at least at some curvature to it, and was probably spherical. This was based on the fact that ships sink below the horizon as the sail out to sea. Eratosthenes determined the circumference of the Earth, thereby also proving that it was round. But the notion of a spherical Earth didn't start with him.