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 collecting habits more than the proliferation of warehouses like this one, where masterpieces are increasingly being tucked away by owners more interested in seeing them appreciate than hanging on walls.

With their controlled climates, confidential record keeping and enormous potential for tax savings, free ports have become the parking lot of choice for high-net-worth buyers looking to round out investment portfolios with art.