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?

The crushing weight of water generates the Underwater Universe's ultimate weapon: Pressure. It has banished humans from much of the world's oceans, only a handful have defied it head on and survived. The deeper man goes, the greater the power of water. The bends, exploding lungs, oxygen toxicity, nitrogen narcosis, and imploding submersibles... Is it possible for anything to survive the deepest place on earth some seven miles down? Answers will surprise. Plunge through five ocean zones, each deadlier than the last, to uncover the mysteries of 'Fatal Pressure'.

The predators of the Underwater Universe have evolved to become our planet's most efficient killing machines. This episode explores the top five deadliest animals that no thinking human wants to tangle with, each more deadly than the next. But there is an unexpected twist, the #1 predator might soon be replaced by another, far more menacing killer as ocean conditions change and traditional apex predators die out.

The Underwater Universe generates three types of deadly waves, each bigger and more destructive than the last: Rogue, Monster and Tsunami. This episode will progress from bigger to bigger waves and explore how these killers are born and what makes each unique. We end the hour with the biggest recorded impact wave in Earth's history: a mega Tsunami over a hundred feet high destroying everything in its path. How did it happen? How did the planet survive? And could we be hit by a wave that big again?