The Sydney-Santiago flight route is one of the southernmost passenger flights. Such a route is impossible if Earth were flat and confirms that Earth is a sphere. It is easy to verify the existence of this route from flight websites.
On a flat Earth, this route should go across the United States. But the duration of the Sydney-Santiago route with a stop in the US is much longer than the non-stop route because Earth is a sphere, and this route does not go anywhere near the United States.