[kaibee]

> But how do you protect a continuous line of a few thousand km?

Cameras/sensors, and fast response time. You could put a sensor package every couple km. Given the amount of power this installation delivers, it wouldn't exactly break the bank.

[credit_guy]

The bottom of the ocean is a very dark place. Sun light does not penetrate beyond a dpeth of 200 meters. Your cameras need to have lights, and very powerful ones, but they won’t reach beyond, say 500 meters. So you will need many thousands of cameras and probably hundreds of autonomous submersible drones to be on station at all times. This does not come cheap. Leaving aside the fact that it may render the project uneconomical, after a few years of nothing happening, there will be lots of cost cutting pressures. Even without cost cutting, at depth things fail. You will have zones with no suveillance coverage. The problem with such a system is that 99% coverage is about as good as 0% coverage.