Quick googling tells me the typical diameter of an undersea cable is 25 mm. If we assume let's say 15 mm of that is actual fiber (wild guess) and approximate them as a single cylinder that works out to only about 0.03 km^3 of glass... that's a lot more feasible than I was expecting. Comparable to the volume of concrete used to build the Three Gorges Dam. I suppose it's only that "reasonable" because we're talking about a hypothetical 200 exabits/s, puts into perspective just how fast that is.

https://www.wolframalpha.com/input?i=pi+*+%28%2815%2F2%29+mi...