In these times, when everybody is a Skype, MSN or mobile phone text or call away, it's difficult to imagine someone simply vanishing. But, the fact is, they do, and their disappearances intrigue people for years, decades, even centuries afterwards. Take the story of D.B. Cooper, an American who hijacked a Boeing 727 in November 1971. He demanded US$200,000 in US$20 notes and four parachutes. After the plane landed - the passengers were allowed to leave - and he had received his money and parachutes, he had the plane take off again. He told all the crew to go into the cockpit, and then jumped out of the rear door of the plane. He was never seen again. Officially, he is believed to have died in the jump, but books and articles have been written suggesting that he started a new life with the money from the hijacking. In other cases, the circumstances surrounding a disappearance are more mysterious. In October 1978, Frederick Valentich was flying a light aircraft over the Bass Strait - the stretch of ocean that separates southeastern Australia from Tasmania. During the short flight, he contacted Melbourne Flight Control and told them that his engine was not running properly and that there was a strange aircraft above him, making very unusual movements. Shortly before disappearing off the radar, he contacted flight control again, and said: 'That strange aircraft is hovering on top of me again. It is hovering and it's not an aircraft.' He was never seen again, and no trace of his plane was found either. Perhaps the most famous disappearance of all time is the empty ship that was found floating at sea, its entire crew gone. The Marie Celeste was a merchant sailing ship built in 1861. Her first captain died nine days after taking command and he was the first of three captains to die aboard the ship. On December 4, 1872, the Marie Celeste set off from New York to Italy with a supply of alcohol. A month later, she was found adrift at sea, completely deserted. The ship still had six months' supply of food and water on board. To this day, nobody knows what happened to the crew.