DANBURY, Conn. >> Peter Buck, whose $1,000 investment in a family friend’s Connecticut sandwich shop in 1965 provided the genesis for what is now the world’s largest restaurant chain — Subway — has died. He was 90.

Buck, a nuclear physicist who was born in Portland, Maine, in 1930, died at a hospital in Danbury, Connecticut, on Nov. 18, Subway said in a statement. The cause of his death was not disclosed.

At 17, family friend Fred DeLuca had asked Buck how he could make some money to help pay for college. Buck’s answer? Open a sandwich shop.

In 1965, he and DeLuca opened “Pete’s Super Submarines” in Bridgeport, with the priciest sandwich selling for 69 cents.

The duo changed the name to “Subway” three years later and decided to turn it into a chain by franchising — a move that would eventually make both of them billionaires. Forbes estimated Buck’s net worth at $1.7 billion. DeLuca died in 2015 at age 67.

Subway says it now has more than 40,000 locations worldwide, topping McDonald’s and Starbucks.

“We didn’t make a profit for 15 years,” Buck told The Wall Street Journal in 2014.

Asked if he ever thought the chain would grow so big, he told the newspaper, “Well, I always thought we’d get bigger and bigger, but I really didn’t have a certain number in mind.”

As a physicist, Buck was hired by General Electric in 1957 at a laboratory in Schenectady, New York, and worked on atomic power plants for U.S. Navy submarines and ships. He later worked for United Nuclear in White Plains, New York, and Nuclear Energy Services in Danbury, where he made his home, according to an obituary prepared by his family.

He also pursued philanthropy, making significant donations to many organizations including the Smithsonian Institution, to which he gave a 23-carat ruby named after his late second wife, Carmen Lucia Buck, in 2004.