Access is through an old plumber's workshop, where visitor panels dripping with damp and fungus set out what little is known about the cove. This leads to an even darker, mouldier room where the entrance to the cove begins. Here, rough stone steps lead you ten metres underground to the snaking of tunnels and chambers below.The size is surprising, the intricacy of the carving astonishing. And the sense of the mysterious is overpowering.
What is known about the cove is that local blacksmith George Patterson claimed to have hewn the rooms and passages from the rock between 1719 and 1724. Thereafter, his family lived there until his death in 1737. We also know from church records that the Pattersons used one room as a public house to sell alcohol – not unusual at the time.
Although unconventional, this underground home may have provided reasonable accommodation when compared with the state of housing above ground. Down here, the family would have had space – even if less than comfortable. As you wander round the passageways looking into bedrooms where they might have slept on their rocky beds or the dining room where they would have sat at its stone table, it all seems bizarrely homely.Set further back from the living quarters is the "drinking parlour" which would have been hidden behind a wooden door. Beside the door is an alcove where the pub's toilet – or bucket – would have nestled, close to a conveniently placed drain. The parlour itself is a chambered room whose vaulted ceiling affords the room its other title of "the chapel". Around a stone table, like a long public bar, carved benches would have provided seating for the pub regulars. Most remarkable of all is the "punchbowl" hollowed out of the table. This basin has been at the centre of much of the mystery surrounding the cove.
The dispute and the intrigue arise from a suggestion that far from constructing the whole building, Patterson merely inherited an existing structure.
In 1897 FR Coles, Assistant Keeper of the National Museum of Antiquities of Scotland, investigated the cove. His conclusion was that it would have been impossible for one man to have built it in five years. Furthermore he suggested that the stonework showed clear signs of pick-marks, as used by miners, and not the chiselling he would have expected had the blacksmith indeed fashioned himself an underground home.
Mining and Gilmerton have had a long connection. Coal and lime have been found here since the 13th century and the last mine only shut in the 1960s. It is no great leap to suppose that Gilmerton cove was a trial bore – a seam that miners dug out, which came to nothing. It is reasonable to think that Patterson found the caves already excavated and set about building the "furniture" that can be seen today.If you accept that the structure pre-existed Patterson, then the function of the different rooms is open to debate. It may well be that Patterson used the drinking parlour as a pub and the punchbowl held alcohol. But this might not have been the original function.
Look closely and you see mason's marks on the "bar" which might lead to the conclusion that this was a masonic meeting place. Search further and you can glimpse the faint tracing of a carved animal, a cat sitting perched beside the punchbowl. There are some who think the cove was in fact a home to witches, or a coven.
Add more letters and you get Covenanter – and there is a theory that the chapel was the meeting place of people persecuted for their religion, who gathered far from prying eyes to conduct their services in secret.
The punchbowl may have been a baptismal font – the hidden room serving as a chapel for Roman Catholics to baptise their newborn.
Further exploration of the cavern only deepens the mystery. Two small bolt holes - tunnels that shoot off out of the building - have been found. There is speculation that one leads towards nearby Craigmillar Castle. The other is said to head straight to Rosslyn Chapel only a few miles away.
With the revelation of secret passageways – or escape routes – theories topple over each other like dominoes. Into the already heady mix of masons, witches and Covenanters comes more fanciful notions. Echoing around the caves is a distant whisper of a deep and hidden mystery. It is said that the Knights Templar – those fighting Crusader monks – used this place for assignations, entering secretly through the tunnels that run back to Rosslyn.
And as everyone knows by now, the Templars didn't just bring fighting talent when they came out of the East, but something else. Which leads to one final question about Gilmerton.
Could this neglected, half-forgotten cave be the true resting place of the Holy Grail?
If you enjoyed reading this, you may want to read:
A night in Edinburgh's haunted undercity