Forrest Fenn, the eccentric American millionaire who claims to have hidden a fortune in precious jewels and gold, has now released an eleventh clue to treasure hunters looking for his weighty stash.

An 82-year-old art and antiquities dealer in the American state of New Mexico, Fenn claims he wants to motivate people to spend time out of doors and enjoy the natural beauty of their country, rather than spend time indoors, watching television or playing video games.

Fenn said that he tried making the bounty enticing by hiding rare and prehistoric artefacts in the treasue chest to make it "valuable enough to entice searchers and desirable enough visibly to strike awe".

To that end, he says his hidden treasure and the promise of gold will persuade people to indulge in real-life Indiana Jones-like adventures.

His eleventh clue reads: "No need to dig up the old outhouses, the treasure is not associated with any structure."

Forrest Fenn treasure chest
Cover of Forrest Fenn's memoir The Thrill of the Chase

The tenth clue, released on 1 March, hinted that the treasure (valued at $1m; roughly £670m) was hidden 5,000 feet above sea level. And the latest, treasure hunters believe, indicates the gold is buried in his home state - New Mexico.

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According to the Mail, the treasure now includes such priceless pieces of jewellery as a turquoise bracelet, a Tairona and Sinu Indian necklaces, pre-Columbian animal figures, Chinese figurines made from jade and assorted antique jewellery studded with rubies and emeralds.

Real or Hoax?

Meanwhile, speculation also suggests the whole "treasure hunt" is an elaborate scam on the part of Fenn. However, Doug Preston, the author of an adventure novel called The Codex, admitted to having seen the chest.

"I've seen the treasure. I've handled it. He has had it for almost as long as I've known him. It's real. And I can tell you it is no longer in his vault," Preston said, "I am 100 percent sure that he (Fenn) really did go out and hide this thing. I am surprised that anyone who knows him would think he was blowing hot air. It is just not his personality."