Pulling off a hoax is no mean achievement. It takes planning, guile and more than a measure of plausibility. For a good hoax to succeed, people have to first think, "Well, it could be true."

On this April Fool's Day, we remember 10 of the greatest hoaxes of all time.

NO. 10: THE HOTHEADED NAKED ICE-BORER RAT

The hotheaded naked ice-borer rat was said to have first been spotted off the Ross Sea in Antarctica. About 6 inches long at maturity, with a body temperature of 110 degrees (because of a high metabolic rate) and fanglike teeth, these rats purportedly could bore through ice easily; penguins were said to fear their jaws of death.

In the April 1995 issue of Discover magazine, an article speculated that hotheaded naked ice- borer rats were responsible for the disappearance of "heroic polar explorer Philippe Poisson, who disappeared in Antarctica without a trace in 1837."

The great clue to the overtness of the hoax (other than the date, April 1) was the author's byline. Aprile Pazzo is Italian for "April Fool."

NO. 9: THE 'DREAM SEASON' ON 'DALLAS'

Hunky Patrick Duffy, who played upstart kid brother Bobby Ewing in the Ewing family oil cartel at Southfork, deep in the heart of Texas, wanted out of the popular prime-time soap opera so he could pursue a movie career.

Usually, in daytime soaps, someone just disappears in an airplane crash in the South American rain forest — you never really know the person is dead, just in case he or she needs to be reinserted in the script at a later date.

But Bobby Ewing definitely was killed off. He was dead. Yet, after a year's hiatus, Duffy wanted back into the program.

Turns out that his pretty, pouty, prissy wife Pamela (played by Victoria Principal, now a cosmetics queen) had dreamed the whole thing. Bobby wasn't really dead. Not only that, she had dreamed the entire season (1985-86) that the show aired without Bobby.

NO. 8: THE PETRIFIED MAN

This is right up there with Bigfoot and the Loch Ness Monster, credible hoaxes in their own right. And there were many "Petrified Man" hoaxes in history, too. What's surprising (or not) is that Mark Twain was involved in one of the best.

According to noted journalism professor and author Fred Fedler, Twain, at the time a novice reporter in Virginia City, Nev., wrote that "a petrified man, about 100 years old, had been found nearby. Every limb and feature was perfectly preserved ... even the man's left leg, which had evidently been a wooden one."

According to Fedler, many readers believed the story when it first broke in 1862, but it actually had been written as a satire about the local coroner, who also was a justice of the peace. Twain believed the man, who was scripted into the hoax, knew nothing about science or medicine and couldn't tell a real corpse from petrified wood.

NO. 7: THE HOWARD HUGHES AUTOBIOGRAPHY

In 1971, author Clifford Irving and McGraw-Hill, his publisher, announced the imminent publication of reclusive billionaire Howard Hughes' "autobiography." Irving said he wrote it, but that Hughes had commissioned him to do it and had provided all the information in the form of handwritten notes, which Irving showed off.

What lent credibility to the plot was the mysterious demeanor of Hughes late in his life. The former moviemaker and aviator was a recluse.

Yet, Hughes gave a short telephone interview (anyway, people think it was Hughes) to denounce the book, and another author demonstrated that parts of the Irving "autobiography" had been lifted from his own, unpublished biography of Hughes. Also, the handwritten notes said to be from Hughes were shown to be fakes, albeit of a high caliber.

Irving was sentenced to 2 1/2 years in jail for his crime.

NO. 6: NOAH'S ARK FOUND

Believers in at least three world religions accept that the Great Flood as described in the Old Testament happened: a great flood covered the Earth, but because Noah was a good man, he, some of his family and a host of animals were saved when they set sail on an ark that God commissioned and Noah built.

Several sightings of the ark's remains have been reported down the years. But in 1993, CBS aired "The Incredible Discovery of Noah's Ark." It later was exposed as a remake of an earlier film about the ark, and sources for the documentary were shown to be veteran tricksters.

NO. 5: THE PERPETUAL MOTION MACHINE

Everyone knows it takes energy — electric, nuclear or whatever — to run machines. But what if you could invent a machine that, once started, would run forever?

There have been many examples of the perpetual motion machine; one was the "hydro-pneumatic pulsating vacuo engine," also known as the "vibratory engine." A prototype circa 1874 was said to produce enough energy from a quart of water to move a loaded train from New York to Philadelphia, according to the Museum of Unworkable Devices and other sources.

No such machine can work as advertised, though. Why? The most common explanation has to do with friction. Moving parts in any machine will generate friction, and that represents a loss of energy, which will have to be replaced with new energy inputs.

NO. 4: PILTDOWN MAN

This was the "missing link" between man and the apes, and it allegedly was recovered in 1912 near Piltdown Quarry in Sussex, England, by amateur fossil collector Charles Dawson, reports the Institute for Creation Research, among other sources. The Piltdown Man hoax was so successful that its bones were enshrined for years in the British Museum in London. But in 1953 the hoax was exposed: Piltdown Man was a composite of a human skull and the jawbone of an orangutan.

NO. 3: THE TRUTH IS NOT OUT THERE

OK, this is the one a lot of you still believe. It's 1947, the Cold War between the United States and the Soviet Union is on in earnest, and a rancher in Roswell, N.M., finds some debris out on the range. Obviously, a flying saucer had crashed. According to Infoplease, what lended credibility to the reports of a government coverup was the fact that there probably was a government coverup, but of high-altitude spy balloons, not flying saucers. For years thereafter, people kept seeing "flying saucers," and in the 1990s, TV's "The X-Files" propelled the myth, if not outright hoax, to new heights.

NO. 2: INVASION FROM MARS

This is the hoax everyone knows, and it was a good one. Actor-director Orson Welles once hosted a radio program, and on Oct. 30, 1938, he and his fellow players read from an adaptation of the classic sci-fi novel "The War of the Worlds." Welles and company did the performance as if it were a straight news story, with what today would be called "live remotes" from journalists at the invasion site (Grovers Mill, N.J.). This was all done decades before Jon Stewart's fake news program, "The Daily Show," on cable TV's Comedy Central.

As quoted on the Web site www.transparencynow.com, one fake journalist on the original program described the hideous Martians this way: "Good heavens, something's wriggling out of the shadow like a gray snake. Now it's another one, and another. They look like tentacles to me."

NO. 1: SWISS SPAGHETTI HARVEST

According to the Museum of Hoaxes, this is the greatest hoax of all time. "In 1957," reports the museum (www.museumofhoaxes.com), "the respected BBC news show Panorama announced that thanks to a very mild winter and the virtual elimination of the dreaded spaghetti weevil, Swiss farmers were enjoying a bumper spaghetti crop. It accompanied this announcement with footage of Swiss peasants pulling strands of spaghetti down from trees."