After the opening prayer of Canto XI Dante briefly describes the overall process of purgation as similar to how one sometimes dreams of the boulders – likely referring to the sense of old distortions disappearing and healing in a sort of dreamlike state.
Così a sé e noi buona ramogna
quell’ombre orando, andavan sotto ‘l pondo,
simile a quel che tal volta si sogna,
Roughly: “Like this to themselves and to us, these shades were singing good prayers, walking under the boulder, similar to that which one sometimes dreams of.”
The “pondo” as stone/boulder thus becomes a symbol of the whole terrace, and the whole central transition in a life as well – removing the distortions, healing, and opening up to fuller reality and a categorically different experience and participation in Being.
The elements within oneself, slowly being crushed, the faculties gradually rebuilding healthy and aligned assumptions and elements of a new cosmology.
This process itself is really something of a miracle.