Kind of an Underdark map, or doing a cavern system, how about mapping an underground river system. You don't have to include the entire river, as in both its source and outlet, but show us the biome it creates and its path through the rock. Show more than a river inside a great cavern though, show us passages completely submersed in water, show twists, waterfalls (dropping into pits), channels through the rock.

Perhaps create a yucatan river system with areas used for drowning pits near Mayan temple sites. Perhaps create a dwarf region of the underdark and how the river is used to drive water mills and water pumps that supply the under mansion. Lots of ways to go, but let's primarily reflect the river itself.