There is the possibility of significant differences in operations and geology (i.e. rockmass permeability, strength etc.) but it is interesting this type of mining has been done before... Notably before WWI when they were still using hamster driven pumps.
Iron Ore Mines of Bell Island - "The submarine slopes were begun in 1905, and the Nova Scotia holdings were reached in 1909. By 1911, Nova Scotia Steel and Coal owned about 32 square miles of submarine beds, while the Dominion Company held about 8 square miles. By 1951, miners were working three miles out under the sea. The minimum allowable mine ceiling - in this case the minimum thickness of the sea floor which formed the roof of the mine - was 200 feet (~60m). In some places, it was up to 1600(~500m) feet." Reference: http://www.heritage.nf.ca/society/bellisland_mines.html Interesting read for those into mining history.
