A chasm is a world in which the only natural light comes from the rift in the sky. It is a world that stays damp even on the hottest days, a drowned landscape of moss, fungus, and hardy plant life that survives even in dim light.
A chasm is wider at the bottom, perhaps a result of highstorms. The storms cause great floods to crash through the chasms; to be caught in a chasm during one of the storms was certain death. A sediment of hardened crem creates a smooth pathway on the floor of the chasms, but it rises and falls with the varying erosion of the under layer of rock. In some places, the distance from the chasm floor to the plateau above is only forty feet. But in most places, it is one hundred feet or more.
The chasms are a bewildering maze, in which can be found the corpses of the fallen, and their spheres, breastplates, weapons, etc.. Bridge crews are routinely sent into the chasms to scavenge from the bodies and retrieve these items. Finding a valuable sphere would result in a small reward for the crew. However, the highstorms sweep all of these away, depositing them within yet another chasm or chasms, so searching the chasms can be either time-consuming or futile. By the bridgemen, chasm duty is loathed; they aren't able to take any of their own possessions in with them and they are searched thoroughly when they come out. However, this humiliation isn't the only reason why chasm duty is so loathed ... 
Aside from the mosses, fungi and plants that live within the chasms are the chasmfiends, hence their name. Occasionally, chasmfiends stalk the pathways that the bridgemen scavenge ... scarring the walls, scraping away the moss, scoring the stone ... in search of carrion or a suitable plateau upon which to pupate. Encountering one of them is uncommon, but possible.