Experts believe thousands of marks may have been left in the fabric of the building over the centuries.
Stonemasons who build Kirkwall’s imposing landmark are among those who left their signature behind but more cryptic markings, some which could point to grave locations, can also be found.
The most recent marks were made during World War Two by sailors based in Scapa Flow who scrawled their names and ships into the cathedral’s masonry.
Orkney Archaeology Society is leading a project to record the first record of all the graffiti found at St Magnus Cathedral.
It will work with the University of Highlands and Islands with archaeologists to train up volunteers to document the unique markings.
One inscription claims to have been left by Leif Ericson in the year 1156, but doubt has been cast on the date it could have been left, according to BBC Highlands and Islands.