Deep in the Swiss Alps, next to an old airstrip suitable for landing Gulfstream and Falcon jets, is a vast bunker that holds what may be one of the world’s largest stashes of gold. The entrance, protected by a guard in a bulletproof vest, is a small metal door set into a granite mountain face at the end of a narrow country lane. Behind two farther doors sits a 3.5-ton metal portal that opens only after a code is entered and an iris scan and a facial-recognition screen are performed. A maze of tunnels once used by Swiss armed forces lies within.

Demand for gold storage has risen since the 2008 financial crisis. Many of the wealthy see owning gold as a hedge against the insecurity of banks and a reasonable investment at a time when markets are volatile and bank accounts and low-risk bonds pay almost no yield.