Another enduring puzzle is the vaulted archway at the east end of the chapel. In The Da Vinci Code, Sophie, the cryptographer, has an epiphany while gazing up at the forest of small, elaborately carved boxes that jut from the ceiling. Some think the pattern they make contains the encoded notes for a piece of music. Legend has it that anyone who deciphers and plays the tune will be shown the "treasure" of Rosslyn. On the day I visited, a young man from Florida was staring intently at the ceiling, a well-thumbed copy of The Da Vinci Code in his hand.

"You can really feel this place is hiding something," he whispered."It's just a matter of figuring it out."

The finest masons from all over Europe came to build Rosslyn. Standing in the sacristy, where some of their original sketches are still visible on the walls, you can imagine them at work, chattering in a babble of languages, trading ribald jokes, competing with each other to produce the most dazzling carvings.

The Apprentice Pillar is a beguiling monument to rivalry run amok. According to legend, the master mason charged with carving it traveled to Rome looking for inspiration. On his return, he found that his apprentice had sculpted the pillar into a fabulous Tree of Life, coiled with serpents and vines. The master flew into a rage and struck his pupil dead on the spot. Elsewhere in the chapel there is a carving of a young man with a gash in his right temple, which may be the fallen apprentice.

Some think the Apprentice Pillar actually contains the Holy Grail, perhaps in the form of a chalice holding Christ's blood. Though recent scans found no metal inside, visitors still approach the pillar with awe, running their hands over it in search of secret doors or tapping it for signs of hollowness.

Like Loch Ness with its monster, the Rosslyn Trust is happy to trade on the myths surrounding the chapel. The gift shop sells books about the Holy Grail, the Templars, and the Freemasons, as well as copies of The Da Vinci Code.

The downside is that staff face a stream of visitors with more enthusiasm than knowledge. Many forget that The Da Vinci Code blends fact and fiction. One example: Brown invented a Star of David etched on the chapel floor. "We tell people it's not there, but some won't take no for an answer," sighs ­Stuart ­Beattie, the chapel's director. "They want us to pull up the carpets to prove it."

Even if you have not read The Da Vinci Code, Rosslyn is the perfect place to unleash your inner quester. The carvings are an exquisite tease, always hovering on the edge of surrendering their ancient secrets. And if the Holy Grail eludes you, there is always the consolation of a nice cup of tea in the cafe.

Other stops on the Da Vinci Code Trail
The novel opens with a murder and a coded corpse at the Louvre in Paris, and this is where the trail starts. Retrace the characters’ steps to the paintings by Leonardo da Vinci, especially the Mona Lisa, which will never look the same again to those who’ve read the book. Then head across town to the Sober Church of Saint-Sulpice. Follow the thin brass strip that, in the novel, helps the albino monk uncover a secret chamber at the foot of an obelisk. Be warned that staff take a dim view of grail hunters trying to find loose tiles in the floor. Next stop is Temple Church, built by the knights templar in London in the 12th century and scene of a remarkable double-cross in the book. Barristers from the nearby courts scuttle past, dressed in black robes and wigs, lending a rarefied air. The round nave is lined with effigies of knights carved in purbeck marble. A mile away stands Westminster Abbey, where the sinister teacher reveals his identity in the novel. Its majestic, cavernous interior holds the remains of monarchs, statesmen, poets, musicians, and scientists. Finally, join the other code fans studying the orb on the marble tomb of Sir Isaac Newton.