There was one guy who designed what is actually an elegant experiment, placing several vertical surfaces, signs basically, with holes in them in a straight line a distance apart at the same elevation above sea level. If the Earth is flat, one should be able to see straight through all of the holes in the signs; if the earth is round, the holes will curve down and away. He observed that they curved down and away. Then of course proceeded to make up bullshit about how his own test was invalid.