Caves are great. They can be made in several ways. They are a Lighting challenge. A Scenic's dream.
One element that always bothers me if it is not addressed, however, is the dirt floor. It is a very common oversight to finish the Scenic walls, dump barrels of dirt on the floor, rake it out and call it a day.
What you are left with is a very unnatural intersection between the floor and the walls. This will make your cave look faker than faux.
There's a really good n' dirty way to fix it.
With a hudson of scenic wash dampen the edge first; the dirt can then be rubbed into the lower 12-24 inches of the cave wall and feathered out. As the wash/dirt dries it will blend in nicely with the cave wall, and goes a long way to ease the dirt/wall transition, leaving a very natural look.