After the Greeks realized the world was actually round and not a flat square, it became a well-known fact that it was safe to sail the seas without tipping over the edge. Heck, even Christopher Columbus was able to set out on his quest to "India" and was able to help discover North America centuries ago. However... there's always that ONE or two conspiracy theorists that have something else to say about the matter.
They might want to re-think that logic. But honestly, if they believed this was a solid reason why the world is flat... there's probably little reasoning with this person.
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