How come some really big waves are not so deadly and used [View All]

for surfing??? I mean I have seen pictures of guys surfing what appears to be at least a 50 ft wave (eg Mavericks). Are these not vicous killers because they are crashing on land used to these waves and there is usually a backdrop of cliffs (probably created by these waves over a long period of time) that shield the rest of the area? Eitehr way, how could such waves be surfed if it only takes 20 foot waves to kill and destroy so much in a single instant?

I initially wondered if the waves were crashing with a 500 MPH velocity, but that theory was debunked earlier today while listening to NPR - their geologist said that the waves go 500 MPH far out at sea, but slow down to a normal speed when they get close to shore.