As palladium prices break above $1,500 an ounce for the first time, the market is wondering how much of the metal is stockpiled across the globe.

Concerns over a supply deficit helped the white metal, used mainly by automakers to reduce emissions in gasoline-powered and hybrid cars, to surge more than 60 percent in the past six months.

Yet the level of inventories -- held by traders, customers, banks and other investors -- remains a mystery. The only visible figure is the holdings in exchange-traded funds, which may no longer be enough to bridge the projected supply gap.

London-based researcher Metals Focus Ltd. estimates that above-ground stocks of palladium fell to 13 million ounces by the end of last year. Heraeus, a precious metals refiner, said various sources have estimated a range of 10 million to 18 million ounces, which equates to roughly one to two years of demand.

“The question is: at what price will the holders sell?' Heraeus said in a research note this week.

A lot of that inventory originated from top producer Russia since the 1990s, before being stockpiled in countries from the U.S. to Switzerland. While the exact figure is unknown, old Soviet stocks held by the Finance Ministry’s Gokhran mineral depository were depleted five years ago.

Russia’s top miner MMC Norilsk Nickel PJSC has sold about 1 million ounces over the past two years from a fund it set up in 2016 to buy palladium from third-party sources including the nation’s central bank, according to Anton Berlin, the company’s head of analysis and market development. Nornickel may disclose what, if anything, remains when the company reports its annual results next week.

While not all of the global inventory would be immediately available, Heraeus concludes that “a chronic shortage is unlikely.”