The disappearance of Dan "D.B." Cooper is one of the great unsolved mysteries of the 20th century. 

 
On November 24, 1971 a man calling himself Dan Cooper purchased a one way ticket on Northwest Orient Airlines Flight 305 which flew from Portland, Oregon to Seattle, Washington. He appeared to be just an average guy. No one could of known what Mr. Cooper had planned.
  
Shortly before the 4:35pm take off, a note was passed to a stewardess asking for four parachutes, $200,000 in unmarked bills, and "no funny stuff." The note also mentioned that Cooper had a bomb. The stewardess first thought he was passing her a note asking for her phone number, giving her his number, or otherwise expressing an interest in her and she simply pocketed it. If she thought there was nothing worse than getting a picked up by a middle-aged passenger, she was soon to learn otherwise. She didn't actually read it until they left the ground and turned off the seatbelt signs. When the stewardess finally read the note, she made the flight crew aware of the situation and they immediately contacted airline and airport officials.
  
The plane circled above the Seattle airport until a call from the FBI at 5:24 p.m. indicated they were able to come up with the parachutes and money. Though the bills were indeed unmarked, the FBI had used the circling time to use a high-speed copy machine to commit images of all 10,000 $20 bills to microfilm to aid in creating a list of the serial numbers later.
  
When the plane landed in Seattle, Mr. Cooper allowed all passengers, who strangely were unaware they were being high jacked, to leave the plane. The flight crew and a single stewardess were made to stay behind and once the money and parachutes were delivered, Mr. Cooper then demanded to be flown to Mexico. Knowing that he would be jumping out of the back of the plane, he also demanded that the pilot fly with landing gear down, flaps set to 15-degrees, to not exceed 170 miles per hour and not to fly above 10,000 feet. 
  
The plane was refueled, and even though it was full, with the demand the flaps be down, there was no way the plane would have enough fuel to make it to Mexico. Mr. Cooper agreed to a stop in Reno, Nevada to refuel. 
  
After take off for Reno, Mr. Cooper asked the stewardess how to lower the rear stairs in the back of the 727. He then ordered her to close the curtain in first class behind her and to remain with the rest of the crew in the cockpit for the remainder of the flight. 
  
A final peek before shutting the curtain revealed a skyjacker attempting to tie something around his waist. It was the last anyone ever saw of the man called Dan Cooper. In all likelihood, he was tying the 21-pound bag of money to a tether in hopes it would land first and give him some indication that the pitch-dark landing was about to occur.
  
At around 8:00 p.m. the pilot noticed a red warning light which indicated a door was open. If this had happened at 35,000 feet, no one would have been left alive to notice the blinking red light but Cooper had wisely requested an altitude of only 10,000 feet. Though the warning light did not indicate which door had became ajar, the crew correctly deduced that the aft passenger staircase in the tail section of the plane had been forced open by the skyjacker.
  
The alarmed pilot asked over the intercom "Is everything OK back there? Is there anything we can do for you?" "No!" shouted Cooper and he was never heard from again. 
   

FBI Profile of D.B. Cooper

  
Research written up by Rose 

Information gathered from Super70s.com