In recent years an artist named Simon Fenoulhet found a tunnel in Cardiff city centre.
While he was reluctant to divulge the exact location, he did concede it was around Greyfriars Road.
He believes the tunnel, which he climbed through in 2014, may be related to a 13th century friary that once stood where the city centre is now.
A book called Cardiff Castle, published in 1923, mentioned a hidden underground passage which is thought could be the same tunnel.
It noted the tunnel was “discovered on the south bank of the Dock Feeder where it passes through the Greyfriars enclosure. The opening passage is 2ft 3in wide and on entering it, a tunnel runs westward towards the castle”.