A metal door set into a mountain in Switzerland offers a way out for fine art investors forced to pay over the odds to insure their collections. The doorway leads into a disused military bunker, one of several being sold off by the Swiss government and whose echoing, climate-controlled chambers, once used to stockpile munitions, are being put forward as ideal storage space for works of art.

The 57-year-old bunker near Lake Lucerne, marketed as a site to store valuables and on offer at 386,000 Swiss francs ($417,000), could relieve a huge concentration of costly paintings at the world's biggest fine art vault in Geneva, the storage option of choice for wealthy buyers worldwide.

    That warehouse, prized for its high security and its location in Geneva's tax-exempt freeport zone, holds art worth about $100 billion and has space to accommodate more.

But insurance underwriters, fearing losses if the facility were hit by a fire or an art heist of the kind that struck Rotterdam's Kunsthal museum earlier this month, are raising the cost of insuring any more paintings under its roof, or even refusing cover altogether.

"If you are a prudent insurer you want to make certain that you know how much risk you are exposed to at a certain location," said Nick Brett, underwriting director at AXA Art Insurance, the world's biggest specialist art insurer.