When the world watched blurred images of Neil Armstrong first stepping on the moon 40 years ago, it marked what most people came to believe was the greatest achievement of the 20th century.

But for conspiracy theorists, the whole thing smelled of an elaborate hoax from the beginning. Hoax-theory advocates say the Apollo program was a mountain of lies that amounts to the greatest conspiracy of all time.

NASA says that's nonsense.

But on occasion, the conspiracists have gotten under the skin -- and in the faces -- of NASA officials. In 2002, leading conspiracy theorist and filmmaker Bart Sibrel confronted astronaut Buzz Aldrin at a Beverly Hills Hotel, calling the second man to walk on the moon "a coward and a liar and a thief."

That prompted the spry then-72-year-old Aldrin to punch the much larger Sibrel, 37, square in the face.

Forty years later, what drives the doubters?

Hoax Theories Take Root

Almost since the first moon landing on July 20, 1969, skeptics have pored over details and data of the six manned Apollo moon missions and sought to expose what they consider flaws in the evidence.

Watch a slideshow of some of the most popular conspiracy theories.

Doubters make piles of arguments: the photos and videos contain irregularities; 1969 technology was too primitive to support such a journey; the astronauts couldn't survive radiation exposure.

Conspiracists suggest the Apollo astronauts remained in Earth's orbit -- or perhaps weren't in space at all. One theory holds that they were holed up somewhere during the mission and that the moon-landing footage and photos were conjured on a secret soundstage loaded with Hollywood props.

Perhaps the founding father of the moon-hoax theories was writer Bill Kaysing, who often pointed to the lack of stars in NASA's moon-landing photos (NASA advocates say that argument is easily explained as a matter of camera settings). Another early leader, writer and inventor Ralph Rene, became a conspiracist who also argued the government was behind the Sept. 11 attacks. Both have died in recent years.

Conspiracy theories that took root in the years after the first landing were fueled in part by the 1978 movie "Capricorn One," which tells the story of a faked NASA mission to Mars.

With the dawn of the Internet, hoax theories got renewed life in cyberspace, where it can be harder to recognize fact from fiction. The hoax believers got a boost in 2001 when Fox TV ran a one-hour special called "Conspiracy Theory: Did We Land on the Moon?" The segment prompted NASA to offer an unusual debunking in the form of a Web page.

NASA typically does not respond much to conspiracy theorists. In a statement for this story, the agency said it is "disappointed that conspiracy theorists ignore basic science principles to diminish arguably the greatest feat in the history of human exploration. It insults the dedication and the sacrifices made by hundreds of thousands of people who were a part of the Mercury, Gemini and Apollo programs."

The Government 'Lied'

Perhaps the leading advocate of the moon-hoax theories now is Sibrel, who says he has spent $500,000 and 10 years researching the moon landings. Sibrel, whose movies are for sale at Moonmovie.com, said he'd bet his life the events were faked. He argues in his film "A Funny Thing Happened on the Way to the Moon" that the hoax was necessary for the U.S. to intimidate Cold War enemy Russia and save face around the world.

"The fact is that they did lie, and having gotten away with it emboldened them, and they continue to manipulate the people of this country," Sibrel said.

Sibrel weaves together dozens of arguments to create a tapestry depicting a massive conspiracy.

Sibrel said the biggest smoking gun is NASA video he says he obtained from the Apollo 11 mission that includes audio of a third party prompting the astronauts on when to talk, which he says is evidence of a produced film. The video, featured in Sibrel's movie, also shows what he says is a badly faked shot of the Earth when the astronauts were supposedly halfway to the moon.

Even most of the thousands of people involved with the Apollo program were kept in the dark about the hoax, Sibrel said. Only people at the top, including U.S. presidents, know about it, he said.

"It was much easier to fake than people realize. All the press coverage was controlled by the government, and there were only three eyewitnesses," Sibrel said. "The best way to fool the world is to fool the media."

Sibrel gained some attention for his altercation with Aldrin (Sibrel said he now regrets using such harsh language with the astronaut). He filmed the confrontation for a movie in which he tracked down aging Apollo astronauts and asked them to swear on a Bible that they went to the moon. Only a few agreed to swear. Armstrong refused, telling him, "Mr. Sibrel, knowing you, that is probably a fake Bible."

Sibrel dismisses all the Web sites that debunk hoax theories, many of which make detailed point-by-point arguments.

"It doesn't matter that they have an answer for everything," Sibrel said. "It doesn't make them right."

Actually, it does, and NASA has a lot of evidence to prove its case, agency supporters say.

'People Love A Conspiracy'

NASA came back from the moon loaded with evidence, including 12 astronauts from six missions who walked on the moon, reams of data and media and 842 pounds of moon rocks unlike anything found on Earth.

The conspiracists often leave out key information as they make their arguments, said astronomer Phil Plait, perhaps the most high-profile moon-landing conspiracy theory debunker.

Plait has been debunking moon hoax theories for years, but said it was difficult to choose the most outlandish one he'd seen.

"It's like trying to choose one of your favorite kids. Which one is the most ridiculous? The answer is none. They are all equally ludicrous," Plait said.

The enormity of a massive, 40-year conspiracy just isn't believable, Plait said.

"You are talking about NASA keeping the most massive secret of all time. And these were the same bumbling keystone cops who could not shoot pictures properly? So which is it?" Plait asked.

A small segment of the population believes in the hoax theory because "people love a conspiracy," Plait said. "But when you look at it critically, it all falls apart."

'Myth-Making'

It’s not hard to understand how people doubt the moon landings in a country with a long history of mistrusting government, said Dr. Howard McCurdy, a space policy expert at American University.

McCurdy points to Yellowstone National Park, which many doubted could exist when early explorers brought back descriptions of it. Early explorers doubted an undiscovered southern continent (Antarctica) could exist, he said. And no one believed the first climbers of Mount McKinley when they claimed to have reached the summit, McCurdy noted.

"We haven't been back (to the moon) in 40 years -- that encourages the same kind of myth-making," McCurdy said.

But McCurdy added that some of the conspiracy theories that stretch to explain an elaborate hoax are more outlandish than the Apollo program achievements.

"The trick to spinning a conspiracy theory is taking a little kernel of truth and weaving a fable around it," McCurdy said.

Borrowing from author Carl Sagan, who in his book “Contact” employed Occam's Razor -- the rule that given competing theories, the simpler one is true -- McCurdy said, "The 'simpler explanation' is that we went to the moon."

• Slideshow: Moon-Landing Conspiracy Theories