It is trivial to prove the earth is round though which makes it trivial to prove the earth is not flat (which does makes it impossible to prove the earth is flat).
Travel to any large body of water. Mark a boat with lines showing how high above the water each line is. Have somebody else pilot the boat out to sea. You observe that the lines show the boat as if its sinking. They observe the lines staying at a constant height. It's because the earth is curved and the water between you is blocking sight of the lines.
Travel to any large body of water. Mark a boat with lines showing how high above the water each line is. Have somebody else pilot the boat out to sea. You observe that the lines show the boat as if its sinking. They observe the lines staying at a constant height. It's because the earth is curved and the water between you is blocking sight of the lines.