Coincidence? A look at conspiracy theories

Almost immediately after TWA flight 800 exploded off the coast of Long Island, N.Y., killing all 230 people on board, theories began to emerge about what caused the disaster.

The crash happened when the Internet was still in its infancy, and it became one of the first news stories fueled by online speculation and conspiracy theories. For some, the findings of the NTSB investigation - which concluded after a four-year probe that the crash occurred because a spark ignited the center fuel tank - were never satisfactory. Among the theories: the plane was shot down by a missile -- either as a terrorist attack or a Navy exercise gone wrong.

These theories were recently reinvigorated by a group of former investigators who are convinced a TWA Flight 800 was brought down by an outside explosion and have petitioned the government to reopen the probe, citing new evidence.

Read more:TWA flight 800 gets another look 17 years later

Moon landing

About 6 percent of Americans believe that the Apollo moon landing was faked, according to a 1999 Gallup poll. The theory rests on the belief that Cold War-era pressure to beat Russia to the moon led NASA to fake an elaborate mission on a soundstage, well within the Earth's atmosphere.

Believers in this theory cite details from photo and video footage - such as a flag that appears to flap in the airless environment of space and a rock that seems to have the letter "C" inscribed on it. Some theorists have used Stanley Kubrick's 1968 masterpiece "2001: A Space Odyssey" as proof that the technology existed to create a realistic-looking space environment on film, while others have gone as far as to claim that Kubrick himself had a hand in the hoax.

Roswell

On July 8, 1947 Public Information Officer Walter Haut of the Roswell Army Air Field in New Mexico issued a press release indicating that a flying saucer had been found in the desert. By the next day however, the Army had changed their tune, stating the object had actually been a weather balloon, but it was too late.

Rumors that a UFO had crashed in the area, and that an alien body was recovered from the wreckage, spread fast. Ever since, Roswell has been synonymous with UFO theories and has been referenced in countless movies and films, including the television series "Roswell" about teenage aliens. The city of Roswell has embraced their status as UFO ground zero and hosts an annual festival that attracts visitors from around the world.

Read more: 65th anniversary of Roswell

CIA and Crack

The use of crack cocaine in the U.S. soared in the mid-80s. During that time, a number of journalists connected the Contras

JFK Assassination

The assassination of President John Kennedy on Nov. 22, 1963 has perhaps spawned the most conspiracy theories in U.S. history. The official probe by the Warren Commission in 1964

Chemtrails

Streaks of frozen vapor left in the atmosphere by jet engines are called contrails and they are a common site in modern society. Believers in a popular conspiracy theory, however, refers to these man-made clouds as "chemtrails," and believe the government has deliberately laced the vapor with unknown chemical agents.

Websites supporting the chemtrails theory suggest large-scale plans to use chemicals for everything from weather modification to population control and some have even suggested that agricultural corporations are using the sinister technology to destroy organic farming. Believers in this theory have blamed illnesses, including higher-rates of Asthma and COPD, on chemtrails.

Princess Diana's death

It wasn't long after the tragic death of Princess Diana and fiance Dodi Fayed in 1997, that rumors began to surface that dark intentions were behind the couples' fatal car crash. French officials concluded that the couple's driver, Henri Paul, who was also killed, was at fault for the crash. According to reports, Paul was impaired by drugs and alcohol and was driving at nearly twice the speed limit when he lost control of the car in a Paris tunnel.

But theorists, most notably Fayed's father, Billionaire Mohamed Al Fayed, believe that Diana was murdered. Rumors have persisted that she was killed for being an embarrassment to the royal family and Al Fayed has famously claimed that the death was a conspiracy involving Prince Phillip, Prince Charles and even the CIA. Al Fayed has also stated that he believes Diana was pregnant with Dodi Fayed's child at the time of her death.

Read more: Diana conspiracies will live on