Undersea Cables

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.

The Transatlantic Cable
In the book |"The Laying of the Cable" by John Mullaly we read journal entries showing that segments between various points on the supposed globular earth required surplus cable.

In the following entry 16% surplus cable is reported:

Another segment of the cable laying showed a 15% surplus:

Additional quotes similar to the ones above may be found in the work as well.

Southern Hemisphere Cables
Of interest, there are relatively few submarine cables which travel East-West in the Southern Hemisphere.

https://www.submarinecablemap.com/