But for Ashley Cowie - who has spent the best part of a decade trying to work out its meaning - the carving has huge global significance for Scotland when it comes to the history of ancient navigation.
"What is down there is an example of a lost system for measuring time and distance involving both latitude and longitude. It's a priceless mapping treasure."
This navigational teaching board - if that's what it is - forms the basis of Cowie's new book, The Rosslyn Matrix, which presents his case for Rosslyn Chapel having a cartographic explanation.At first glance, the mysterious carving looks a bit like a miniature electricity pylon with a latticed construction of uprights and grids. At the top is the outline of a misshapen cup which has a five-pointed star on one of the sides. Inside the cup shape, stacked on top of each other, are four diamond-shaped lozenges of different lengths and widths.
The crypt is part of an older structure on top of which the 15th-century chapel was built. It was used as a workshop during the chapel's construction and the scratching on the south wall seems to have been dismissed as a workman's sketch for one of the roof pinnacles.
"I can say with confidence this carving does not represent a pinnacle, or any three-dimensional church spire design," says Cowie. "This becomes obvious when you unravel the geometric layers."
Which is where he has the edge on most of us. Because where we see an electricity pylon or a badly drawn cup, Cowie sees a multi-layered, geometrically defined, mathematical template.
Perhaps it is easier to establish what Rosslyn is not before trying to explain what it is – which is why Cowie devotes the first 40 pages of his book to debunking myths.
"It is not a copy of Solomon's or Herod's Temple. It has no 'Grail Trail' link with a Jerusalem-based meridian, and mathematical analyses of original ground plans show it to be an unfinished collegiate church."
Not a knight in sight.
"I challenge anybody to show me a Templar symbol in there," he says, speaking with the righteousness of the newly converted.
"Yes, I went down all the usual roads and got involved with all the esoteric stuff, Knights Templar included. I've come out the other end with my feet now firmly on terra firma."
A decade and more has passed since his love of local history led to him brushing the dirt off an old crest on a ruined Caithness farmhouse lintel. Researching into the background of what turned out to be the engrailed cross of the St Clairs led to a fascination with the skills of the family of ancient master builders and their craftsmen.
Determined to remain unfazed by the current hype, he examined William St Clair's 15th century Rosslyn Chapel from a practical perspective.
"As Earls of Orkney and Caithness, William and his predecessors learned their navigational skills from their Norwegian overlords who were experienced explorers," Cowie explains.
The carving is what he believes to be the chapel's most enduring legacy. If correct, it must surely make a hugely important contribution to Scottish scientific history.
Getting him to explain its importance in a few short paragraphs is impossible, and the following synopsis will no doubt cause him much anxiety.The lozenges at the top are ancient symbols showing degrees of latitude. They're based on shadows recorded at solstice sunrises and sunsets producing different shapes at each degree of latitude. Tall, thin lozenges relate to northern places like Norway, which the top lozenge on the carving represents; wide and squat signifies a Mediterranean band, as in the bottom one. The middle two lozenges are the latitudes for Orkney (northern
Scotland) and Rosslyn (southern Scotland).
The pylon-shaped grid is a longitudinal slice of 15 degrees (1/24th of the 360 degree globe) and its vertical central line is an ancient meridian. The cup shape isn't a cup, or grail chalice. It's an astronomical drawing of the orbits traced by the morning and evening star, Venus.
All this was important to ancient navigators for synchronising dates, times and locations.
Cowie has certainly opened up new avenues of approach, and as far as Rosslyn Chapel is concerned they're happy to hear him out.
"It's interesting and refreshing to have a new perspective on Rosslyn," says Stuart Beattie, who oversees fundraising efforts of Rosslyn Chapel Trust.
Whether Cowie's discovery will change the way we think about ancient navigators, or just be swallowed up with all the other mysteries surrounding Rosslyn, only time will tell. But it seems certain that despite years of research this chapel still has a number of secrets left to unravel.
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