What is the theme in The Lower Depths by Maxim Gorky?

The theme of life in the lower depths as like prison or hell circulates throughout the text. The title is immediately telling; these scenes from Russian life are about what is dark, underground, buried. Lower depths, as opposed to height or the heavens, suggest hell by sheer proximity, and the miserable lives of the residents make this connection clear. Lack of light also suggests the dark side, or hell. So does lack of meaningful work, or progress, as suggested by Kleshtch's ceaseless scraping at a lock that can never be mended. In Acts II and IV, Bubnoff and Krivoi Zoba sing a song about being in prison, never seeing the sun rise or set, and this sets a tone for life in this underground cavern. Even in Act III, which is set outside, a brick wall is described as blocking out "the heavens."