On February 2, 1870, the Cardiff Giant was finally revealed as a fraud in court. In fact, both of the fakes were declared fake. The original one and the plaster copy that PT Barnum had been showing – while declaring the original fake to be fake. Confused yet?
The Giant was the creation of a New York tobacconist named George Hull. Hull, an atheist, decided to create the giant after an argument with a fundamentalist minister named Mr. Turk about a passage in Genesis that stated that there were giants who once lived on earth.
The idea of the petrified man did not originate with Hull, however. In 1858 the newspaper Alta California had published a bogus letter that claimed that a prospector had been petrified when he had drunk a liquid within a geode. Some other newspapers had also published stories of supposedly petrified people.
Hull hired men to carve out a 10-feet-long, 4.5 inches block of gypsum in Fort Dodge, Iowa, telling them it was intended for a monument of Abraham Lincoln in New York. He shipped the block to Chicago, where he hired a German stonecutter to carve it into the likeness of a man and swore him to secrecy. Various stains and acids were used to make the giant appear to be old and weather beaten, and the giant's surface was beaten with steel knitting needles embedded in a board to simulate pores. Then Hull transported the giant by rail to the farm of William Newell, his cousin, in November 1868. He had by then spent $2,600 on the hoax.
When the giant had been buried for a year, Newell hired two men, Gideon Emmons and Henry Nichols, ostensibly to dig a well. When they found the Giant, one of them has been attributed to saying "I declare, some old Indian has been buried here!".
Between the time the Cardiff Giant was "discovered" and the revelation that it was a fake, many intelligent people fell for the hoax. A man who watched the odd hoax unfold before his eyes, Andrew Dickson White, wrote:
The current of belief ran more and more strongly, and soon embraced a large number of really thoughtful people. A week or two after my first visit came a deputation of regents of the State University from Albany, including especially Dr. Woolworth, the secretary, a man of large educational experience, and no less a personage in the scientific world than Dr. James Hall, the State geologist, perhaps the most eminent American paleontologist of that period.
On their arrival at Syracuse in the evening, I met them at their hotel and discussed with them the subject which so interested us all, urging them especially to be cautious and stating that a mistake might prove very injurious to the reputation of the regents, and to the proper standing of scientific men and methods in the state, that if the matter should turn out to be a fraud, and such eminent authorities should be found to have committed themselves to it, there would be a guffaw from one end of the country to the other at the expense of the men intrusted by the State with its scientific and educational interests. To this the gentlemen assented, and next day they went to Cardiff. They came; they saw; and they narrowly escaped being conquered. Luckily they did not give their sanction to the idea that the statue was a petrifaction, but Professor Hall was induced to say: "To all appearance, the statue lay upon the gravel when the deposition of the fine silt or soil began, upon the surface of which the forests have grown for succeeding generations. Altogether it is the most remarkable object brought to light in this country, and, although not dating back to the stone age, is, nevertheless, deserving of the attention of archaeologists. 
At no period of my life have I ever been more discouraged as regards the possibility of making right reason prevail among men.
As a refrain to every argument there seemed to go jeering and sneering through my brain Schiller's famous line:
"Against stupidity the gods themselves fight in vain." 
There seemed no possibility even of suspending the judgment of the great majority who saw the statue. As a rule, they insisted on believing lt a "petrified giant," and those who did not dwelt on its perfections as an ancient statue. They saw in it a whole catalogue of fine qualities; and one writer went into such extreme ecstatics that he suddenly realized the fact, and ended by saying, "but this is rather too high-flown, so I had better conclude." As a matter of fact, the work was wretchedly defective in proportion and features; in every characteristic of sculpture it showed itself the work simply of an inferior stone-carver.
The Cardiff Giant has been called the greatest hoax in American history. Well, that's probably not true any longer if it ever was. There are lessons here for people who have declared that they "understand the science" of a number of subjects.
The "real" Cardiff giant is on display at the Farmer's Museum in Cooperstown, New York. I have seen it myself. (Photograph here.)