Dante and Virgil cross into a swampland. This is the circle of the wrathful and sullen (or bitter). The wrathful mangle, tear, and attack each other in the muck and slime.
The swamp is the river Styx, another river in the underworld from classical mythology. The black muck is also bubbling at the surface in many places. Virgil explains to Dante that these bubbles are from the souls of the sullen who lie entombed underneath the slime.
In this instance, Dante does not show pity for the damned, indicating that perhaps he is making moral progress in understanding divine justice. Dante and Virgil then leave the swamp and approach a high tower.