When passengers boarded Swiss International Air Lines Flight 17 from Zurich to New York City last weekend, they probably had no idea that the cargo hold beneath their feet was also functioning as a bank vault, carrying tens of millions of dollars.

An American bank was sending cash from its Swiss office back home — something that occurs with surprising frequency, even in the digital age. While money can be wired, credited, debited and transferred around the world in a split second, there has never been more American currency in circulation, and the amount keeps growing. And that cash moves around frequently, often carried discreetly in the belly of commercial airliners.

Someone, it seems, was wise to the shipment on the Swiss plane.

By the time the cash reached its destination, a Federal Reserve center in the New York area, over $1 million dollars had vanished.

“We are investigating the apparent disappearance of $1.2 million in U.S. currency,” James Margolin, a spokesman for the Federal Bureau of Investigation, said on Tuesday.