jisco

It's not the case here. On their website, google states: ... Dunant is the first long-haul subsea cable to feature a 12 fiber pair space-division multiplexing (SDM) design ...

Multi-mode fibers are not feasible for long distance transmission. For long distance communications, using suggested approach, may be better to use multi-core fibers.

I don't think it's a good idea.

If there will be any fissure in the steel container, then the water we put on top for sure will get there and in the end, that hot contaminated water will raise at the top of water column.