Cavern diving is the exploration of permanent, naturally occurring overhead environments while remaining within sight of their entrances. It differs from cave diving in that, while cave divers may penetrate thousands of meters, cavern divers generally go no further than 130 feet from the surface.
I found this explanation on:
What Is Cave Diving?
which should help.
Additionally, cavern divers keep the entrance clearly in sight at all times, and use a guideline so that, should sight of the entrance be accidentally lost, divers can immediately regain it.