Villagers in north west France are offering prize money to anyone able to crack a 230-year-old inscription found on a rock.
Historians and decoders have been left baffled by the 20 lines of text, written in both French and Scandinavian characters, which were discovered on rocks hidden in a cave near the town of Plougastel-Daoulas five years ago.
The rock, which is one metre tall, has been dated to around 1789, but the two dates 1786 and 1787 are also etched into the rock. Some believe the text could relate to forts located nearby.
One part of the carving is thought to read: "ROC AR B... DRE AR GRIO SE EVELOH AR VIRIONES BAOAVEL... R I OBBIIE: BRISBVILAR... FROIK...AL."
A £1,750 reward is now on offer for anyone who can help crack the code.
Plougastel mayor, Dominique Cap, who set the competition to run until November, said he has already received thousands of submissions.
He added: "We've asked historians and archaeologists from around here, but no-one has been able to work out the story behind the rock.
"So we thought maybe out there in the world there are people who've got the kind of expert knowledge that we need.
"Rather than stay in ignorance, we said let's launch a competition."
Some characters carved into the rock are upside down, or reversed French letters.
However, the Scandinavian characters like 'Ø’ a vowel from the Scandinavian languages of Danish, Norwegian, Faroese and Southern Sami, have also been inscribed.
Municipal Councillor Michel Paugam, who is in charge of local heritage, said: "There are a lot of words, they're letters from our alphabet, but we can't read them, we can't make them out."
Véronique Martin, a local official who is leading the appeal, said the dates "correspond more or less" to the construction of forts and artillery batteries, including the adjacent Corbeau fort, built to protect the nearby harbour city of Brest.
After the public submit their suggestions, a jury will decide on the most likely interpretation of the inscription and award the €2,000 prize.
"This inscription is a mystery and it is for this that we are launching the appeal," Martin said.
Agencies contributed to this report.
