"Dark as a Dungeon" is a song written by singer-songwriter Merle Travis.
It is a lament about the danger and drudgery of being a coal miner in a shaft mine.
It has become a rallying song among miners seeking improved working conditions.
During this live performance, one of the prisoners in the background was laughing, and Cash started to chuckle.
When he finished the song, Cash made a comment that was largely repeated, somewhat out of context, by Joaquin Phoenix in the 2005 Cash biographical film Walk the Line: "I just wanted to tell you that this show is being recorded for an album released on Columbia Records, so you can't say 'hell' or 'shit' or anything like that.