Shortly after taking off, "Cooper" passed a note to the stewardess: "I have a bomb in my briefcase. I will use it if necessary. I want you to sit next to me. You are being hijacked."
So began the tale of what has become one of the most brazen and legendary (some have even said heroic) crimes in American history...
Cooper (often referred to as "D.B. Cooper") showed what he purported to be a bomb (some red cylinders in his briefcase that later turned out to be harmless), demanded $200,000 in unmarked bills, and four parachutes: two loaded in the front of the plane and two in the back. After the plane landed at Seattle-Tacoma the demands were met and the Boeing 727 took off again.
At around 8 p.m. Cooper bailed out of the rear of the plane, holding onto his newly acquired satchel of cash. Along with the parachute he wore the business suit he wore when he boarded the plane: seemingly no protection at all against the elements, but Cooper by all accounts was cool and confident nonetheless.
Hurtling himself into pitch black night with freezing rain and driving wind, Cooper was never seen again.
Here's the Wikipedia entry about Dan Cooper, where you can find out much more regarding his infamous skyjacking along with the various theories that have cropped up over the past four decades.