The pull of the moon and spin of the earth generates mighty forces in the oceans that sweep relentlessly around the planet. Four of the Underwater Universe's most powerful tides and currents pose the greatest potential threat to mankind: the Gulf Stream, the extreme tides of England's Morecambe Bay, the killer rip currents of Kauai and finally--biggest of all, one that helped wipe out over 80 percent of all species some 251 million years ago, the Great Ocean Conveyor. How did an ocean current trigger the planet's most devastating mass extinction event? And could it happen again?
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