Occasionally, an art theft reads like the plot from an Ian Fleming thriller in which half-mad billionaires pay slick international thieves to steal famous paintings for their private pleasure. In reality, experts said Monday, most art is stolen to earn a quick buck in an international black market that is second in size only to the drug trade. The priceless art stolen Sunday from the Isabella Stewart Gardner Museum is too famous to be sold anywhere, raising the spectre that the pieces could be held hostage for ransom for a fraction of the hundreds of millions of dollars the art world says the paintings are worth. The true scale of the art underworld is difficult to gauge, but investigators say art thievery is increasing as prices for works on the legal market soar. ``There's a lot of it going on,'' said Robert W. Holmes Jr., a Boston attorney specializing in fine art transactions. ``Ninety percent of stolen art is never recovered. There's obviously a market there.'' Rarely, one end of the market is held up by someone who decides to commission the theft of a painting that cannot be legitimately acquired. One such theft occurred in 1983, when thieves stole Italian Renaissance masterpieces, including Raphael's self portrait, from the Hungarian national museum in Budapest. Investigators found that a sophisticated band of Italian art thieves, armed with cutting tools and high-tech alarm-busting devices, executed a perfect break-in and vanished with the works. An extensive dragnet eventually snared the thieves in their home country before they could get the paintings to Greece where, it was later found, an olive oil baron had paid a handsome sum for the sake of having the works hang in his home. ``There aren't many thieves with that sort of expertise,'' said Robert Volpe, a former New York City police detective who worked on the Hungarian case and is now a private consultant tracing stolen art around the world. ``They were capable of stealing anything.'' But most of the black market in art is in lesser-known works usually ranging in value from $100,000 to $300,000. Priceless works such as the Vermeer taken from the Gardner Museum are simply too famous to be sold anywhere. ``It's incredibly stupid to steal a Vermeer or a Rembrandt,'' said Steven Keller, a private security consultant who was brought in as security director at The Art Institute of Chicago after a major theft of three works there by Cezanne in 1979. ``He (the Gardner thief) has got some things that are too hot to handle,'' Keller said. Keller said lesser-known works and prints that could be sold more easily were more valuable to thieves than masterworks that couldn't be fenced. ``You might be able to sell say, a lesser-known American artist's painting in Europe,'' Keller said. ``It's harder to spot them.'' Often, thieves who can't sell stolen art underground hold it ransom for insurance money, a scenario several investigators and curators said was likely in the Gardner theft. Last year, for instance, thieves stole three paintings by van Gogh, demanded a $2.2 million ransom and returned one of the works. The others were later recovered and four people suspected of the theft were arrested. ``It happens, and it happens frequently,'' Holmes said. ``But like any hostage situation, it's got to be outside the public eye, so often these cases go unnoticed.'' Keller, noting the relative worth of the art stolen from the Gardner Museum, said, ``If I were the thief of the Gardner paintings, I'd ship the Vermeer and the Rembrandt back to the museum and go for a ransom on the smaller works.''