No matter how high you go in the sky — whether it is in an airplane or in a high-altitude hot-air balloon — the horizon (where the ground meets the sky) will always rise to your eye-level. This is a proof that the earth is flat plane. How far does the plane extend? No one knows.
If the Earth was a sphere, then the higher you go in altitude, the more you would have to look down in order to see the horizon. But as it is in reality, when we go higher we do not have to look down — the horizon always stays in the same position relative to us.