According to the ball earthers (that is most people) the horizon is caused by curvature on a round earth.
An observer on the ground with eye level at h = 5 ft 7 in (1.70 m), is supposed to see the horizon at a distance of 2.9 miles(4.7 km). For an observer standing on a hill or tower 100 feet (30 m) in height, the horizon is supposed to be seen at a distance of 12.2 miles (19.6 km).
So, take a look at this.
“This is the short version of a longer video I will put up showing the results of my trip across Lake Michigan, wherein we saw Chicago the whole time for nearly 46 miles. This one just gives the highlights of the journey. In the longer video, I will explain more about the trip, the observations we had during it and my more recent conclusions based on the past year’s worth of research.”