It's a theory that many have signed up to, most lately writer Dan Brown in his bestselling book the Da Vinci Code. Thousands of avid fans who have read the novel now harbour some suspicion that the Holy Grail is, indeed, somehow linked to Midlothian’s Rosslyn Chapel. So much so, visitor numbers have shot up - in July a record 9029 people visited the 15th-century chapel - a 96 per cent increase on last year. In fact the Grail is a miniature industry in itself, spawning books as well as films and tourist booms like that at Rosslyn. The Grail has been a powerful symbol of Christianity for more than 2000 years and the search for it has captured the imagination of generations, both Christian and non-Christian. There are even theories about what the Grail itself is - some believe it was the cup that caught the blood of Christ at the crucifixion; others, that it was the cup from which Jesus drank at the Last Supper. Some even believe it is a metaphor for a bloodline running from Christ himself. Telling chivalric tales of heroic knights on Grail quests, medieval romance writers and poets made the myth popular, while films such as King Arthur and of course, Indiana Jones and the Last Crusade are eager to exploit the mystery surrounding it. Tonight, a documentary on Channel Five explores the theories around what the Grail is - and what might have become of it. One possibility is that Rosslyn Chapel, the spiritual home of the Knights Templar, a group of crusading monk-knights who defended Jerusalem, is the secret resting place of the Grail. The Templars’ purpose, when they were formed in the 12th century, was to protect pilgrims heading for Jerusalem, which had been captured by Christian crusaders in the 11th century.