Q. If a cable company put down a cable its length would have to be longer than predicted (by round earth geometry) if the world were flat. If somebody put down a bunch of cables and found that they were longer than they'd expected, wouldn't they tell somebody?
A. But the cables are always longer than expected. It's just explained by underwater currents, soil irregularity, winds and errors in placement, et cetera. And somewhere in that is lost a mistake caused by a slight misunderstanding of the Earth's shape