[semi-extrinsic]

Rigid steel pipe is the only feasible way, I believe. Current longest subsea pipeline (Nord Stream) is 760 miles.

Here's how it's done (it's pretty darned fascinating):

https://youtu.be/EyrdjqEiTZc

[neltnerb]

Yet there's the whole thing where the end points are on tectonic plates moving away from one another...

Okay, maybe a little flexible would be good!