Local historian, Gordon Cook sent me this extract from a book written in 1887.
It tells of a forgotten cave on the Bothwell side of the River Clyde, sealed with a gate in Victorian times, and interestingly was large enough for 50 men to get inside, and was allegedly decorated by art!
I found this most interesting! I have NO idea where this is although there were several quarries on the Bothwell side in the 1850’s, directly opposite the Clyde Braes. I was once told that a gated entrance to “tunnel” existed on the Bothwell side below the castle, protected by the castle gardner in the early 20th Century. However, authorities decided to fill over with soil and grass over to stop attempts being made to enter.
On the basis of this heresay evidence and the steep rocky slopes there, I would say it may be around the castle area, but nothing is shown on any map.