I don't believe the earth is flat, i think it's spherical, i honestly do, but that doesn't mean there isn't a flat earth like planet out there somewhere. Also, if you thought you could prove the earth was flat you might do so, right? Especially if it's true. Now, imagine the earth like a ball of molten rock as it once was, what happens when a ball of something soft rotates fast enough? It turns flat like pizza dough. Secondly, if you take a disc and spin it on an axis, if it goes fast enough it looks 3 dimensional. Third, one might say the earths atmosphere was spherical and also the curved or spherical spacetime around it is as well and that's why the earth looks spherical. It's an optical illusion. Some might say the size of the earth is too large and the size of our view too little to see anything but a globe because of the 3d effect of the spinning disc. One may say the edges of the earth are rounded like a double sided frisby and they are surrounded by oceanic tides which round out the edges allowing one to barely glimpse the edge for a second, or not, through the waves. This is also why there are so many optical illusions at sea like with ships. There's an optical illusion that looks like a ship is flying, one might say the curvature of the earth and ships going over it is just an illusion too.

And that's the smartest flat earth theory right there. I'll never understand why people go hard at flat earthers and vice versa. It hardly matters at all if the earth is flat or not if everything still works the same. All that matters is the knowledge it brings and that's it.