On Boxing Day 2004 the world was shocked by one of the worst natural disasters of all time. The cause of so much devastation in Indonesia was the most powerful kind of earthquake on the planet - a megathrust. Scientists have discovered that a megathrust earthquake could happen right off the coast of North America. The source of this threat is a huge geological structure called the Cascadia Subduction Zone. Millions of people live within its danger zone, which encompasses the major cities of Seattle and Vancouver.Horizon tells the story of how Native American legends, ancient Japanese texts and a long-dead forest have convinced scientists that the Pacific Northwest is in danger. The programme looks at what damage this earthquake, and the tsunami it unleashes, will cause, and what is being done to prepare for the inevitable.