If the earth is flat, how thick or thin is it? What is on the other side of the flat earth? How do you know the earth is flat if you don't know how flat it is? After all, we claim to know the circumference of the earth:
http://geography.about.com/library/faq/blqzcircumference.htm
Does the atmosphere wrap around the edge of the flat earth to the other side? Can we dig through to the other side? Does anyone live on the other side of the flat earth? If so, why haven't we made radio contact with them?
Why aren't there any other flat planets, or for that matter flat asteroids, flat moons, flat comets, flat stars, etc? Why would the earth be flat while every other planet, moon, star, etc is spherical?
How could the earth be flat? What are the natural forces that could possibly have produced such a shape? Every object in the observable universe over a few hundred miles in diameter is at least roughly spherical in shape, due to its own gravitational force. Why would the earth be any different? Gravity must necessarily be false for the earth to be flat, considering the earth's mass. Gravity requires an oblate spheroidal earth. If you insist that gravity is not a proven fact, then what do you have to replace it with?
Many proofs can be offered to demonstrate a spherical earth. For example, the earth appears as a disc on photographs taken from space, regardless of the vantage point. The only geometric solid which looks like a circle from any direction is a sphere.
A 100-foot-tall ship that is 15 miles away is not visible. That's because it is blocked by the curvature of the Earth. As it approaches, it "rises." First the tip of the mast is visible, then more and more of the ship comes into view as the ship gets closer:
Earth curvature line of sight:
These are the phases of the moon as seen in the Northern Hemisphere:
These are the same phases of the moon as seen in the Southern Hemisphere:
In the Northern Hemisphere the sunlit part of the moon moves from right to left:
In the Southern Hemisphere the sunlit part moves from the left to the right:
Also, in the Northern Hemisphere, the stars appear to rotate counterclockwise around the North Star, Polaris:
However, in the Southern Hemisphere, the stars appear to rotate clockwise around Sigma Octantis:
All of these things, taken together, are only possible if the earth is a sphere.