[jcranmer]

The "advantage" of deep-sea mining is that there are no neighbors to complain about the environmental effects of mining, and--since it's largely proposed in international waters--you get to choose whose environment regulations you follow.

The disadvantages are more numerous: the commercial viability of the extracted ore is extremely unclear; the seafloor is very poorly mapped, and with poor visibility, you could easily drive a seafloor rover into or over a cliff without seeing it; environmental effects are largely unknown [1]; the international authority meant to help guide these efforts has put a moratorium on it until these questions can be answered (which, given how long it's taking to answer them, has led many companies interesting in deep sea mining to advocate for ignoring it entirely).

[1] Although anyone who's had much of a thought about it would probably hazard that "insanely destructive" is the most likely outcome. Still an open question if deep sea mining is less or more destructive than our current mining techniques.