Thanks. 1/2 https://www.youtube.com/watch?v=Xg0FfxHPGXs They are amateurs and they've shown the curvature in their video. 3 You've never seen droplets of water on an apple The water isn't flat. Or a water barometer? Or water ball in a zero-g airplane? 4&5 Gravity is pulling towards the center of the mass of the Earth. 6 Ships and the horizon. 7 if the project is large enough or need to be very precise, yes you have to calculate the curvature of the Earth. It was factored in during the construction of the Golden Gate Bridge. 8 If you know you elevation along the route, it's not complicated to dig 26 feet below sea level. 9 That book doesn't even exist. 10/11/12 The line goes along the curvature of the Earth. 13 With the lamp at the summit of Desierto (2390 feet) you can see it in Ibiza at an elevation of 1046 feet, which is easily achievable considering the highest point in Ibiza is 1558. 14 Pretty much the same thing, 3 peaks are high enough Foel Cwmcerwyn, Cerrig Lladron, Foel Feddau. 15 Gravity, air density, and auto-pilot. It was fun, albeit repetitive on certain points. From your point of view, what was convincing in that video?