Re James 51
"Do submarine optical cables have repeaters? I can see how land based systems could be upgraded to take advantage of this but there might be a bit of a lag in adding it to global networks."
Modern submarine cables have analogue amplifiers as their submerged plant - they are protocol agnostic and so long as the signal fits within the amplifier's gain band they will work.
What this means is that in order to increase a cable's capacity you just need to replace the landing station equipment - and if you're lucky this might mean no more than replacing a transponder with a higher capacity one working on the same grid spacing (50GHz between lambdas) eg., replace a 2.5Gb/s transponder with a 10, 40 or 100Gb/s one.