Mr. Fenn, who is now 86, came up with the idea while suffering from cancer in 1988, but he recovered from that illness and 20 years later decided the time was right to launch the quest.
The antiques dealer drove out to the Rocky Mountains and buried the 22-pound chest, which contained around US$2 million worth of gold and jewellery.
Mr. Fenn has said the treasure could be in New Mexico, Colorado, Montana or Wyoming, is located above 5,000 feet but below 10,000 feet, is close to some pine trees and not in a man-made structure.
The main clue to its whereabouts was a cryptic poem called The Thrill Of the Chase.
The second verse of the poem reads: "Begin it where warm waters halt, And take it in the canyon down, Not far, but too far to walk, Put in below the home of Brown."
That led many treasure hunters to begin their search along the banks of the Rio Grande river in New Mexico.
Tragically, Paris Wallace, a 52-year-old pastor from Colorado, vanished while searching for the treasure trove and his body was found along the Rio Grande in June.
His death came 18 months after Randy Bilyeu, 54, also from Colorado, went missing while searching in the Rocky Mountains of northern New Mexico. His remains were only found in July 2016.
New Mexico State Police's chief, Pete Kassetas, has called on Mr. Fenn to end the hunt, which he described as "nonsense," before someone else died.
"He's putting lives at risk. It can be avoided. People are coming from other states and other parts of the world to find this elusive treasure that may or may not exist with very few clues and they are underestimating New Mexico," Mr. Kassetas said.
Mr. Wallace's body was found seven miles downstream from where he was last seen and he is thought to have been washed away by torrents of water.
His wife Mitzi confirmed he had been searching for Mr. Fenn's treasure.
When Mr. Bilyeu died last year, his wife accused Mr. Fenn of having faked the treasure, but Mr. Fenn insists it does exist out there somewhere and has even said treasure hunters have been within 200 yards of it.
Last week more than 100 people attended an event called Fennboree, which was designed to be a celebration of the treasure hunt.
Cynthia Meachum, who has been searching for the treasure since 2013, said she believed Mr. Wallace was searching in the wrong place and added:
"In my opinion, if you can't take your kids there, you're in the wrong place."
In an interview with Mr. Fenn which was posted on the mysteriouswritings.com website last week he warned treasure hunters to stay away from dangerous areas.
"Please don't ever overextend yourself. I was 80 or about when I hid the treasure and it was not a difficult task. I will soon be 87 and I could go back and get it if I were so inclined," he wrote.
The American West is full of buried treasure legends.
In 1998 Las Vegas gambling tycoon Ted Binion died mysteriously. Sheriff's deputies later found a 12-foot deep vault in Pahrump, 60 miles away, which contained six tons of silver bullion.