The center was built at a cost of $51 million to contain what one expert called “the memory of humanity” — 250,000 paintings, drawings, statues and other objects from across the world and spanning nine millennia.

The works belong to the Louvre but unlike the French museum’s stars — Mona Lisa, the Venus de Milo or the Winged Victory of Samothrace — they are held out of sight in storage rooms. Now, this literally priceless stored collection — the Louvre says it would be impossible to estimate its worth — is being transferred to a modern conservation center in Liévin, northern France, where it will be held for future generations.

Safe house: reserves at the Louvre Conservation Center, in Liévin, France.

The operation, without precedent in the Louvre’s history, has thrown light on to an issue that curators rarely want to address. Why do museums — the Louvre and many others — keep so many works out of the public eye?

Containing 250,000 paintings, drawings, statues and other objects from across the world and spanning nine millennia.

The Louvre has about 35,000 objects on display in the former royal palace that was turned into a museum after the Revolution. It may feel like a lot to visitors, but it represents just 5.6 percent of its overall collection. Some of the rest are on loan to other museums. Most are stored, notably in the museum’s basement, or in 60-odd warehouses scattered around the French capital.

Officials say they are held out of sight for various reasons: because they are light-sensitive and cannot be exhibited for long, because they are a notch down in historical importance compared to the objects on display, or because tastes and fashions in the art world have changed.

Their transfer follows an interior ministry warning that the 150,000 or so items in the Louvre’s 108,000-square-foot basement risk being damaged by the muddy waters of the Seine the next time Paris floods, which happens once a century on average, the last time being in 1910.

The Louvre’s Arc

The Louvre initially wanted to establish a storage site in Paris before deciding that land in the capital was too expensive and opting instead to build a 200,000-square-foot center in Liévin, next to a provincial branch of the museum opened in the old mining town of Lens in 2012.

Some of the Louvre’s curators were aghast at seeing their treasured collections moved to a region 125 miles from Paris better known for beer and football than for Egyptian antiquities or Renaissance art. “It’s very difficult to understand. They’re cutting the museum in two,” said one.

Jean-Luc Martinez, the chairman of the Louvre, told Arte, the Franco-German television channel, there was no choice. “If we took the decision to externalize our stored collection, it is because we could not have saved all of it [in the event of a flood]. It is not a question of choosing between good and evil. It is a question of choosing between an evil and a lesser evil.” Proponents say the conservation center, designed by Roger Stirk Harbour + Partners, the British architecture firm, and inaugurated a year ago, offers perfect conditions for artworks, with a roof covered in vegetation to ensure a constant temperature, spacious storage rooms and purpose-built trolleys to wheel them in and out.

On the move: this mosaic of the Great Mosque of Damascus has already been transferred to Liévin.

Brice Mathieu, the director, hopes to complete the transfer by 2024. More than 130 lorries containing tens of thousands of objects had made the journey so far amid tight secrecy to avoid attracting the attention of criminal gangs. The unmarked vehicles leave the capital at random intervals, with no one except for a handful of specialists and a few police officers in the know, and their drivers under orders to avoid stopping until they reach their destination.

Among the objects that have already left Paris are The Toilet of Esther, a 1738 painting by Jean-François de Troy, the French master, two three-ton ancient Greek statues of Dionysus and Heracles and 1928 reproductions of the mosaics of the Great Mosque of Damascus.

The items risk being damaged by the muddy waters of the Seine the next time Paris floods, which happens once a century on average.

One advantage of the transfer is that it has enabled the Louvre to discover exactly what it had in storage. Like many of the world’s best-known museums, it had been collecting objects over the years and putting them in boxes that have often been left untouched for decades.

Take, for instance, the hundreds of fragments that had been sitting wrapped in newspaper in a wooden crate ever since being brought back from an archaeological dig in Iran in 1908. Unwrapped for the purpose of the transfer, they turned out to form a 6,000-year-old Neolithic vase. A lump of stone also puzzled curators until they realized that it was the missing shoulder of the 4,000-year-old statue of the Elamite Goddess, Narundi, which has been in the Louvre for more than a century.

The Parisian institution has now identified and photographed its entire collection. Privately, curators suggest that other museums around the world — including some in Britain — still have only a sketchy idea of their collections.

Yet Michael O’Hare, emeritus professor of public policy at the University of California, believes that curators are wrong to stash 90 percent or more of their artworks away from the public gaze, as is often the case. “Lots of major museums’ stored art should be out where people can see it: regional museums, schools, public libraries, hospitals, city halls, and yes, private collections,” he said. “Sell some, rent some, lend some. The purpose of art is for citizens to engage with it; for museums to have it is in service to this goal, not instead of it.”

Lost and found: a missing piece of a statue of the Elamite goddess Narundi, discovered in a box during the move.

Emilie Girard, the deputy chairman of the French branch of the International Council of Museums, disagreed, pointing out that under French law, museums have a mission to preserve, restore, study and enrich their collections, and only afterward to exhibit them. Ms Girard said: “The Impressionists were rejected at first and it was only because museums bought them at the time that we are able to exhibit them today.” Nevertheless, she said that in recent years, some museums had tried to open their stored collections up to the public. The Museum of European and Mediterranean Civilizations in Marseilles, where she herself works, is a case in point, with 4,000 people visiting its Center of Conservation and Resources every year.

There will be no such policy in Liévin, where the Louvre’s 250,000 strong collection will be open to academics but not to the general public. Suzanne Keene, formerly head of collections management at the Science Museum in London, said an opportunity was being missed. “If they are setting up a center like this and now allowing for public access, which they could do, then I think it’s a shame.”

Adam Sage is a longtime Paris correspondent for The Times of London