The curvature of the earth can be demonstrated by looking across a body of water and observing that objects at great distance disappear over the horizon, even though the intervening surface is “flat”. This has been observed for centuries by sailors at sea, but as most people today never have the opportunity to sail the ocean a more accessible demonstration is needed.
Presented here are two land-based observations, using immense North American lakes as the body of water. In both examples, the far shore of the lake and the buildings thereon are over the horizon, and in both cases the bottoms of those buildings are not visible, hidden by the curvature of the earth.
Looking at Chicago across Lake Michigan:
Looking at Toronto across Lake Ontario: