For a long time the people of the city Tjanath would dispose of their criminals by placing them into a cage, then slowly lowering them into a pit that they perceived as being somewhat bottomless, from this pit emanated constant moans that they thought were the slowly dying people they had sentenced to this punishment that they affectionately called The Death
The truth is however is that they were dropped into an underground river (possibly an underground tributary of the River Iss) situated in a massive underground cavern. Here un-mined radium particles give off a dim light, and a unique ecosystem survives (possibly thrives) with unique plants and animals life not common upon barsoom. Including Fish that swim within the river, and the Great White Lizard that hunts down those sentenced to The Death.
Down the river continues above ground in the Valley Hohr.