The Pantheon.
Okay here's a bit more: the Pantheon, when viewed from a distance, looks perfectly straight. The floor, the steps, the columns. However, when viewed up close, the proportions begis to change. The floor takes a bowed up appearance, as do the steps. The columns look bowed around the center. But they are straight! No. They aren't. The Greeks actually built the Pantheon, along with other buildings, like this. A perfectly flat line appears almost bowed down, or sagging, from a distance. To counter this, the Greeks built buildings, at least important ones, with a bowed shape. How is this relevant. Take a flat line. Any old flat line... Oh! The horizon! It.... Oh wait.... From sea level, it looks flat. Now, the only way to get a flat line is to.have a curved shape... Oh but it has to be flat right? Oh... Oh dear... From even a few thousand feet up, as your prospective changes, and you get further from the globe, you begin to see the curve, slight though it may be. The further up you go... It more it curves. Well. This ended poorly. I really thought I had a flat earth proof. All well. At least the hyper accurate gyroscope experiment didnt show a 15 degree drift........ Oh.......