While we're on it, care to be more close minded? Atleast show some response/argument.
Magnetism: It is caused by molten rock moving inside the Earth, creating friction that causes magnetism. If the Earth is flat, compasses wouldn't work. If they had a giant magnet, it would have to be so huge and powerful that it would move very large metal objects, such as cars in the northern hemisphere.
Volcanoes: Where is the lava coming from? How does lava circulate underneath a flat Earth and erupt?
Earthquakes: The Earth would not be solid since there are Earthquakes. It would be completely broken up.
The Oceans: They wouldn't move. With a flat Earth, they would have no spinning motion, but instead only a vertical motion, causing them to be held in place.