Tonga is 10,882 meters (35,702 ft) deep at its deepest point. By the time someone gets around to grabbing that thing, I seriously doubt that a few Kg of weapons grade fissionable material will be relevant.

Theoretically speaking, Lec, Someone could grab it now if they had the right equipment... they'd just have to locate it first.

That's not to say that it's really relevant now, though.

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hrmm if they could pinpoint it's location they many be able to cable down a remote sub with a MASSIVE electro magnet on it. It'd be difficult but possible. Course there might not be enough steel in the bomb to pick it up and who knows what other obstructions you'd face.