Edinburgh is well-known for its subterranean network of passageways, vaults and caverns, but the discovery in 1928 of a tunnel bored into the south face of Calton Hill was a surprise to all.
Exposed by a rock fall, it was clearly visible from the Canongate and nearby districts and caused great excitement amongst residents.
READ MORE - 11 incredible Edinburgh buildings we can't believe were demolished
But the unexpected find posed a number of questions: how old was it, who created it and why? There was only one way to find out.
A team of specialists, led by City Architect Ebenezer James MacRae, was sent in to survey the hard-to-access site, which was located near Jacob's Ladder on the cliff face below the Old Royal High School and Regent Road.
A trawl through the city archives informed the team that quarries had once been active around Calton Hill, but the origin of the tunnel appeared to be unconnected.
Gaining entry into the tunnel was not a task for the faint of heart, as The Scotsman newspaper reported, with the team making their way to the site by "slippery scrambling and shaky planks".
After removing rubble from the mouth of the tunnel, the team ventured inside to learn that it led to a large cavern.
Sign up to our Edinburgh Live nostalgia newsletters for more local history and heritage content straight to your inbox
The report continued: "[The tunnel] runs a long way into the living rock, and the date and object of its making were at first obscure.
"It ends in a fairly roomy vault or cavern, the roof of which descends on each hand so low as to be impassable."
As to the exact purpose of the tunnel and adjoining vault, there was a clue beneath the team's feet.
A mosaic of brickwork on the floor of the cavern suggested there was a connection with the construction of the tunnel for the Edinburgh and Berwick Railway driven through Calton Hill almost a century earlier in the 1840s.
In order to shore up the railway tunnel and prevent a potential collapse, 19th century engineers had created a second tunnel above so that bricks could easily be laid for the building of a reinforced arch.
However, due to the wedge shape of the stone bricks, or "voussoirs" and the increasing difficulty of laying them as they approached the tunnel's centre, the engineers required access from above in order to drop the innermost elements of the arch into place.
The Scotsman report concluded: "The rock of the Calton Hill is not of a kind that can be trusted to be self-supporting, especially when exposed to the vibration of passing trains.
"It was therefore necessary to build the true tunnel of brick, inside of, and clear from the uncertain rock.
"The "voussoirs", being wedge-shaped, must be built in from above, and this was easily done till the advancing halves of the long vault came so close as to make this impossible.
"A small tunnel was therefore pierced from the face of the rock, and the cavern was provided, so the work, filling in the final gap, was completed."
