Interesting idea, I hadn't thought of that possibility. The only thing is, the underwater island had the Dharma Initiative houses on it which would date the island to being submerged in or after 1977. The age of the Atlantis myth (or reality) is much older of course, so the time frame is off, unless perhaps the island itself travels through time somehow? Or do you mean it would be a modern day Atlantis?
My question about the underwater island is why would it be underwater?
If you assume the hydrogen bomb was detonated by Juliet then the underground blast would be powerful, but I don't think it would sink the entire island underwater (and it looked like it was very deep underwater too). I would think a 1950's era bomb would blow that part of the island apart instead and there would still be a percentage of the island that wasn't destroyed above sea level.
But then again I'm no Daniel Faraday, so who knows, maybe it would end up relatively intact a couple hundred feet underwater.
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