Yeah, I'm the MEMB fairy, sprinkling that link in the comments of any story about submarine cables. But you beat me to it this time!

I consider it my small contribution to the world to make sure as many nerds as possible get to read that story.

I assume while he was doing research for Cryptonomicon?

I read the entire article. It does not go into detail about the deepest parts of the ocean. The closest it gets is when it talked about the calculations required to lay cable from the UK to Africa, but this is still close to shore (I remember seeing a diagram showing how the cable angled down a steep slope into deep ocean, but this archived version doesn't seem to have pictures).

Edit: this is what it actually gives you as an answer in that article:

"The answer has to do with slack control. And most of what is known about slack control is known by Cable & Wireless Marine. AT&T presumably knows about slack control too, but Cable & Wireless Marine has twice as many ships and dominates the deep-sea cable-laying industry. The Japanese can lay cable in shallow water and can repair it anywhere. But the reality is that when you want to slam a few thousand kilometers of state-of-the-art optical fiber across a major ocean, you call Cable & Wireless Marine, based in England. That is pretty much what FLAG did several years ago."

Not much of an answer. How the heck do they lay it and control slack at greater than 8000 meters? How do they verify the cable is on the seabed at those depths?

Jets of water are used to bury the cable closer to shore, and if I recall correctly, Google used Kevlar on their latest cables to reinforce them against sharks.

That's in shallow water, not answering his question.