MIB will be stopped by the Island sinking, rather than EM trap???

MIB said he can't fly over the water and he can't leave the island. So being trapped in an electromagnetic circle (which has always seemed kinda unrealistic to me) is not how MIB is defeated. Rather he is killed/contained by being trapped in the ocean on the island. Does anyone remember for sure... the underwater hatch: did Ben say it used to be above ground? And what was the reason he had people still guarding it? Could the island have been partially sunk in the past- such as in '77 when Jughead was hypothetically detonated? Any thoughts?

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