Almost $1.5 trillion of the world's cash[1], with U.S. $100 bills[2] making up a great deal of it, is reportedly unaccounted for. So what happened to the money[3]?

"Literally, a lot of these $100 bills are sitting in bank vaults[4] all over the world," Treasury Secretary Steven Mnuchin[5] told FOX Business' Lou Dobbs[6] during an interview on Tuesday.

Mnuchin pointed to the negative interest rates causing people to turn to American dollars as a solid investment[7].

NUMBER OF $100 BILLS CIRCULATING THE GLOBE SKYROCKETING, THIS COULD BE WHY[8]

"The dollar is the reserve currency of the world, and everybody wants to hold dollars," Mnuchin said on "Lou Dobbs Tonight."[9] "And the reason why they want to hold dollars is because the U.S. is a safe place to have your money, to invest and to hold your assets."

CLICK HERE TO READ MORE ON FOX BUSINESS[10]

Mnuchin said it's interesting that, in a increasingly digital world, "the demand for U.S. currency continues to go up."

VARNEY: THE US DOLLAR IS KING OF THE HILL[11]

"There's a lot of Benjamins all over the world," Mnuchin said.

There are more $100 bills – also known as C-notes – in circulation than $1 bills, according to data from the Federal Reserve[12], which found there are more $100s than any other denomination of U.S. currency. The number of outstanding bills featuring a picture of Benjamin Franklin has about doubled since the start of the recession.

In 2018, the Federal Reserve Bank of Chicago illustrated a correlation between low interest rates and high currency demand, though it...

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