Thirteenth painting in the 'Inferno di Dante Alighieri' series.
The eighth circle of Hell, also know as Malebolge, is a large, funnel-shaped cavern, itself divided into ten concentric circular trenches or ditches. Each trench is called a bolgia (Italian for "pouch" or "ditch").
The third bolgia hosts the simoniacs: the sinners are placed head-downwards in round, tube-like holes within the rock, with flames burning the soles of their feet.