Somewhere in France there lies buried a small bronze statuette of an owl in flight.
Whoever finds it is promised not just the prize of the original sculpture made of gold and silver, but the kudos of having won the world's longest-running treasure-hunt.
Sur La Trace de La Chouette d'Or (The Hunt for The Golden Owl) was an illustrated book first published in 1993, shortly after its author Max Valentin had secretly hidden the owl sculpture at a location in mainland France known only to himself.
At the time there was a vogue for such armchair treasure hunts inspired by the UK's best-selling Masquerade, in which artist Kit Williams laid out a series of complex visual clues for finding a golden hare.
But while all the other riddles, including Masquerade, were eventually solved, the French owl is still out there. And, amazingly, people are still looking.
A quarter of a century on, thousands of so-called chouetteurs continue to pore over the book's 11 enigmas, which can be downloaded free over the internet. They exchange theories in chat forums and meet at annual get-togethers.
There is even an association set up to defend their interests in court.
"I have been looking since August 1993," says Pierre Blouch, a founding member of the A2CO association.
"I remember at the time thinking we had better get a move on, because we were starting three months after the book was published. Little did we realise."
Like many fellow owlers, Mr Blouch has had his pet theories, and at one point could be seen digging up various spots around the city of Bourges.
A retired engineer, he now spends his time sifting through the online literature and hoping for new inspiration.
Deciphering the clues appears to require a combination of scientific knowledge, imagination and a head for codes.
When all is revealed - A Golden Owl riddle
Back to the West, look for the Sentinels
8,000 measures away, they await you
Find them, you will need to inspect them
The 11 enigmas are supposed to lead to a town in France, and then there is a hidden 12th enigma made up of left-over bits of the 11 that pinpoints the exact spot.
With the internet, ideas have been pooled and there is now general agreement over the primary interpretation of several of the puzzles.
It is known for example that the first riddle sets out an order for the subsequent ones, based on the wavelength of colours.
But the web has not been entirely beneficial.
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"The point is that when he designed the puzzles, Valentin only had reference books. The internet drowns us in information that he himself did not have access to," says Pierre Blouch.
Valentin is a revered figure among the chouetteurs. For several years he ran a chatline on France's pre-internet service Minitel, in which he answered questions from hunters.
All these answers have now been compiled like the sayings of a prophet. They are known in the jargon as madits, or "he-told-mes", and form a key part of the online database.
Will the owl ever be found?
Alas Valentin himself is no longer alive to help. He died nine years ago, leaving his secret inside a sealed envelope which is now in the possession of his family.
The other key figure in the story is artist Michel Becker, who drew the book's illustrations and sculpted the owl.
The buried bird is actually a replica of the real one, which is made of gold and silver.
This authentic owl is currently in Michel Becker's possession and four years ago he caused uproar among chouetteurs when he tried to sell it. The courts blocked the sale on the grounds that technically the treasure belonged to the future winner.
The artist played a key role in creating the book but he himself never knew the whereabouts of the owl, which means there is now no-one alive who does.
Some fear that the treasure will never be found - that the spot has been built over or that the clues are just too fiendish to be solved, or even that it was all a hoax from the start.
But true chouetteurs draw inspiration from the great Valentin himself.
In a madit of 1996 he said: "If all the searchers put all their knowledge together, the owl would be found in… two hours."