By compressing the image, we can accentuate the curvature of the horizon due to the Earth being a globe, and not flat.

Methodology:

"The horizon appears flat because the curvature is too small when observed from near the Earth’s surface. However, in some cases, we can exaggerate the curve by taking photographs of the horizon and then magnifying the results vertically".

"The wider the lens and the higher the observer’s position, then the larger the curvature will appear, and the less possibility to be affected by lens distortions".

"A wide-angle lens usually has barrel distortion. Because we cannot know whether the photographer bothered to correct the distortion, it is a good idea to find pictures where the horizon line is right in the middle, where the distortion is minimal."

However:

"If the photo has barrel distortions, then a horizontal straight line below the center point will appear concave. If the result is convex, then there’s a good possibility that the curve of the horizon overwhelms the effect of barrel distortion."