Final answer:
The setting of the 9th circle in Dante's Inferno is a frozen lake called Cocytus, which is unconventional because Hell is typically associated with fire, but in this case represents the coldness of betrayal.
Step-by-step explanation:
The unusual aspect of the setting of the 9th circle in Dante's Inferno is that it is a frozen lake. Contrary to what one might expect from the fiery depths of Hell, Dante describes the 9th circle, where traitors are punished, as an icy expanse. The lake, known as Cocytus, stands in stark contrast to the common idea of Hell being filled with flames and brimstone. In Dante's vision, the traitors are immersed in ice, fixed in various positions, unable to move or speak, depicting the cold-blooded nature of their betrayals.