Some may bring forth the experiment shown in the flat earth FAQ (Bedford Level experiment), this can be disproven with relative ease:
Light is known to defract over water, and bend downwards with the curvature of the earth. This can be shown in this video:
This experiment was done again, (I noticed it was not mentioned) but this time with a line of sight for reference. In the second experiment, the sight line was set at 13 feet above the water. This reduced the effect of atmospheric diffraction and showed the "bump" that is the curvature of the earth. This bump was shown by a middle pole.
A third experiment was done, with 3 poles set at fixed heights in that same channel. In this experiment, the middle pole was found to be almost 3 feet higher than the other poles. This is because the middle pole is slightly higher than the other poles, and light diffraction will cause the pole to look like it is floating, when in reality it is on the water, but light has bent away from it and so it looks higher this proved that the earth has a curve in it, and that the only reason that the boat could be seen at the end of the 6 mile channel was because of light diffraction