For centuries the intricately carved stones of Rosslyn Chapel near Edinburgh have tantalized historians, archaeologists and devoted Christians.
A labyrinth of vaults beneath the 15th-century home of the Knights Templar is reputed to contain dozens of holy relics, including early gospels, the Ark of the Covenant, the fabled Holy Grail and even the mummified head of Christ.
More than 550 years after the first foundation stones were laid, modern technology is about to put the legend to the test.
A group of Knights Templar, successors to the warrior monks who sought asylum from the Pope by fleeing to Scotland in the early 14th century and fought for Robert the Bruce at Bannockburn, are to make a "non-invasive" survey of the land around the chapel. They will use the latest ultrasound and thermal imaging technology in the hope of finding evidence of the existence of the vaults.
"The plan is to investigate the land around the chapel to a depth of at least 20ft," said John Ritchie, Grand Herald and spokesman for the Knights Templar.
"The machine we are using is the most sophisticated anywhere and is capable of taking readings from the ground up to a mile deep without disturbing any of the land.
"We know many of the Knights are buried in the grounds and there are many references to buried vaults, which we hope this project will finally uncover."
Rosslyn Chapel, or the Collegiate Chapel of St Matthew as it was to have been, was founded in 1446 by Sir William St Clair, third and last Prince of Orkney. Built as a celebration of Christ, it is also a monument to craftsmanship.
Bristling with flying buttresses and gargoyles in the highest Gothic style on the outside, the interior is carved with scenes from the Bible, the fall of man, the expulsion from the Garden of Eden, the birth of Christ, the crucifixion and the resurrection.
"Rosslyn is an amazing building. It is a book in stone but, because the symbolism which is written into the chapel is in a medieval language, we haven't even cracked the introduction page yet," Mr. Ritchie said.
Pillars and arches are covered with hundreds of exquisitely carved leaves, fruit, animals and figures. Some curious carvings are said to depict cactus and sweetcorn, chiseled before Columbus set foot in America in 1492.
"There is a whole series of stuff on each section of the chapel, which relates to a different period of time," Mr Ritchie added. "We have to go back to the 15th century and read it with a medieval eye to understand what it all means. All these symbols relate to events in history. It is a book created in stone, which brings in all the apostolic religion, laid over by an astrological form which tracks the seasons, and the plants in the seasons." Both the Freemasons and the Knights Templar claim the ornate stonemasonry of the church is a secret code which, if broken, will reveal the whereabouts of treasures.
One theory suggests that one of the ornate columns, known as the Apprentice Pillar, may contain a lead casket in which is hidden the legendary cup used by Christ at the Last Supper and later used to collect his blood, the so-called Holy Grail. "Once we understand the introduction page we will begin to understand what this book in stone means," Mr. Ritchie added. "We hope to start as soon as possible and get a load of readings from it. We hope to at least find this burial place and maybe the Holy Grail itself."
By Paul Kelbie, Scotland Correspondent