Quai Saint-Michel - "layers"

Layers seen along theSeine

Taken from the Pont Saint-Michel you can see layers from the buildings along the quai, the bouquinistes, the level under the street, the banks of the Seine, and finally the Seine herself.

How have I not noticed the walkway under the street level on this south side of the river?  What are those open spaces in the stone wall, and what lies beyond?  I am thinking of Jean-Valjean and M. Hugo.

Quai Saint-Michel et Pont Saint-Michel

75004, Paris