1) The horizon always appears completely flat 360 degrees to the observer, regardless of how high you go up. Any curvature you think you see is from curved airplane windows or Go Pro cameras and fisheye lenses (which NASA loves to use). The reality is that the horizon never curves because we are on an endless plane. On a globe with 25,000 miles in circumference you would see a noticeable disappearance of objects the further they are as they would be leaning away from you and dropping below the constantly curving horizon!
Completely flat horizon from the stratosphere: http://m.airpano.com/360Degree-VirtualTour.php?3D=Stratosphere-Caucasus
2) The horizon always rises to meet your eye level never no matter how high in altitude you go. Even at 20 miles up the horizon rises to meet the observer/camera. This is only physically possible if the earth is a huge “endless” flat plane.
If Earth were a globe, no matter how large, as you ascended the horizon would stay fixed and the observer/camera would have to tilt downward, looking down further and further to see it.
Horizon always at eye level: https://youtu.be/8twAg6CH2sM
3) The natural physics of water is to find and maintain its level. If Earth were a giant spinning sphere tilting and hurling through space then truly flat, consistently level surfaces would not exist here. There would be a massive bulge of water in the oceans because of the curvature of the earth. If earth was curved and spinning the oceans of water would be flowing down to level and covering land. Some rivers would be impossibly flowing uphill. There would massive water chaos and flooding! What we would see and experience would be vastly different! But since Earth is in fact an extended flat plane, this fundamental physical property of fluids finding and remaining level is consistent with experience and common sense. The water remains flat because the earth is flat!
4) If Earth were a ball 25,000 miles in circumference as NASA and modern astronomy claim, spherical trigonometry dictates the surface of all standing water must curve downward an easily measurable 8 inches per mile multiplied by the square of the distance. This means along a 6 mile channel of standing water, the Earth would dip 6 feet on either end from the central peak. Every time such experiments have been conducted, however, standing water has proven to be perfectly level.