How to disprove the flat Earth theory
Ok, let’s see these photos:
Have you seen them?
Mountains are proof that the Earth is not flat but full of bumps and depressions and holes and other stuff that make it Anything. But. Flat.
The only place where the Earth seems flat is by the sea, and in that case there is water, not earth.
(There are, obviously, other arguments against flat Earth theory, for instance this one, but they are less immediate and exclusive in terms of prerequisites.)