Ships appear to disappear hull-first over the horizon due to Earth's curvature.

One of the oldest and most easily observable proofs of Earth's spherical nature comes from watching ships at sea. As a vessel sails away, its hull is the first part to vanish below the horizon, followed by its mast, and finally its sails or antennae. This phenomenon is entirely consistent with an observer viewing an object moving around a convex curve. If the Earth were flat, a ship would simply appear to get smaller and smaller until it was no longer visible, with all parts diminishing proportionally. The 'hull-first' disappearance is a direct consequence of Earth's geometry.