The drab free port zone near the Geneva city center, a compound of blocky gray and vanilla warehouses surrounded by train tracks, roads and a barbed-wire fence, looks like the kind of place where beauty goes to die. But within its walls, crated or sealed cheek by jowl in cramped storage vaults, are more than a million of some of the most exquisite artworks ever made. Treasures from the glory days of ancient Rome. Museum-quality paintings by old masters. An estimated 1,000 works by Picasso. As the price of art has skyrocketed, perhaps nothing illustrates the art-as-bullion approach to contemporary...