Bad Foundations
The gang were quite fortunate that the prison was already old and in bad shape with weak crumbly walls. The saltwater that ran through the pipes for showering and washing dishes was slowly destroying the pipes and leaking into the prison walls.
Over the years, the salt wore down the cement and eventually caused it to crumble. The prison authorities also kept the water slightly warm to keep prisoners from getting used to the cold temperatures out in the icy waters of the San Francisco Bay.
The Noise
You are probably wondering how so much banging and chipping could be going on without anyone being the wiser. The truth is that the escapees cleverly used prison reform to their advantage. In the 1960’s, it was decided that inmates should be allowed an hour of music each day. Nothing could be heard over the disharmony that ensued.
Morris also played his accordion as loudly as possible whenever he could, and the racket was enough to conceal any noises made by banging or the chipping of cement. The holes in the back of the cells led to an unguarded utility corridor full of pipes which going up and down.