ROCHESTER, N.Y. (AP) — While Scarlett O’Hara stayed cool at home, Dorothy Gale took a year out to go skipping down a digital yellow brick road in a Hollywood film lab.

The recently reunited Technicolor duo could well be spending much of the rest of the millennium killing time with Lassie, Annie Oakley, Tarzan and a canned colony of heroes and villains from the silent-film era.

Thousands of pre-1951 movies captured on volatile nitrate film are kept in frigid, low-humidity vaults in a modest cinderblock building owned by the George Eastman House museum on the piney outskirts of Rochester. Cold storage saves them from rotting away within a lifetime or, worse yet, burning up.

In most cases, these are original camera negatives from the first half-century of motion pictures, classics such as “The Wizard of Oz” and “Gone With the Wind,” the silent era’s top-grossing “Big Parade,” Lon Chaney in “The Phantom of the Opera” and Cecil B. DeMille’s 1923 version of “The Ten Commandments.”

While even the best-kept vintage reels are starting to buckle with age, a beloved movie’s master negative is a sacred object that would cost untold millions to replace.

Much of that value lies in its power to produce the finest-quality copies, be it on 35mm film, Blu-ray DVD or some dazzling format that pops up in, say, the early 26th century.

“I really hope that 500 years from now people can still look at this because it’s wonderful stuff,” Deborah Stoiber, vault manager at Eastman’s Louis B. Mayer Conservation Center, said during an inspection of one of 12 dark vaults kept refrigerated year-round at 40 degrees Fahrenheit and 30 percent humidity.

On the shelves of this climate-controlled celluloid nursing home are prized Technicolor films such as “Meet Me in St. Louis” and “Little Women”; silent gems starring Mary Pickford and Greta Garbo; a Lumiere brothers’ chronicle of President McKinley’s inauguration parade in 1897; and “Olympia,” a Nazi propaganda feature on the 1936 Berlin Olympics shot by Adolf Hitler’s filmmaker, Leni Riefenstahl.

The magical way in which a chilly, dry setting retards shrinking, fading or “nitric melt” inevitably raises concern about the long-term survival of other vulnerable pieces of the world’s film heritage, from safety-based acetate stock adopted in the 1950s to television recordings to flimsy digital-video cassettes.

“Nitrate is turning out to be a historically durable medium that, if stored properly, rivals paper — and well-made paper — as a storage medium for image and sound,” said Patrick Loughney, motion-picture curator at Eastman, the world’s oldest museum of photography and film.

Its out-of-the-way bunker is one of just a handful of nitrate repositories run by major film archives around the country. It isn’t listed in phone books or open to the public. Nor does the plain, single-story building draw the eye on a road where the occasional home is backed by woods or farmland.

On the shelves are 6,600 titles, or 22,836 reels — the oldest surviving negatives or prints dating to the dawn of moving pictures in 1893. The Rochester Institute of Technology’s Image Permanence Institute estimates that climate control can preserve films still in pristine shape for another 800 to 900 years.

Considering how a nitrate fire blinded Alfredo, the projectionist in “Cinema Paradiso,” each vault is rigged with sprinklers and blowout doors.

“Nitrate burns at 16,000-to-17,000 feet per second, dynamite at 24,000-to-25,000 feet,” Loughney said. “It has that disturbing quality of producing its own oxygen, so you can’t put it out with water. If properly stored and handled, then it’s no more dangerous that any other kind of hazardous substance, like gasoline.”

That knowledge came the hard way. Made primarily from sulfuric acid and cotton, nitrocellulose was blamed for disastrous warehouse and theater fires, chiefly in the two decades after Rochester-based Eastman Kodak Co. adapted it from flexible roll film it pioneered in 1889.