We hear about the Jewelled Caverns of Devil’s Mouth in Quest for Knowledge and Aftermath of Armageddon. We don’t get to see them until A View of the Past.
“Behold the Jewelled Caverns of Devil’s Mouth.” Simon gasped. He couldn’t help himself. It was as if a switch had been flipped. He had no idea how they did it. The cavern they were in was vast. It stretched before him, as far as the eye could see. Its walls were crusted with gold and gems of every possible colour. Naked flames were positioned at strategic locations to provide illumination. Each flame was magnified thousands of times by the gemstone reflections. Yellows, greens, blues, reds, purples, the entire rainbow was represented. When he looked up, Simon thought he was looking into a clear night sky. A million fireflies flickered above him like stars in a galaxy.”