Foundations

Limestone, water, and time

Cave diving happens in an overhead environment — water with rock between you and the sky. Understanding how that rock came to be hollow explains most of what you will see down there.

Geology

What karst is

Karst is an area of land underlain by limestone that is honeycombed with sinkholes, underground streams, and caves.

Limestone is a soluble rock, and it easily develops cracks, or joints. As rainwater seeps through those cracks it slowly dissolves the stone. Gradually underground streams and caves form in the rock, and sinkholes develop at the surface.

The processes that form karst topography occur underground. In areas where weathering and erosion have worn away the land above, the limestone ridges, caves and sinkholes characteristic of a karst region become visible.

Even where rain is plentiful, some karst looks dry and barren. In these areas rainwater seeps through a thin layer of soil into the limestone and simply disappears.

The word karst comes from the Karst region of the former Yugoslavia, which is rugged and lined with caves.
— adapted from "Karst", Exploring Your World, National Geographic Society.

Why divers care

The cave is the plumbing

A karst system is an aquifer you can swim through. The passages were cut by moving water and they still carry it, which is why so many cave dives begin at a spring — the water you are entering is on its way out.

That flow shapes everything practical about the dive: which direction is harder, how silt behaves when you disturb it, how quickly visibility recovers, and how heavy rain hundreds of feet above can turn a clear system brown overnight.

Vocabulary

Terms you will hear

Not exhaustive, and not a substitute for a class — but enough to follow a conversation at a fill station.

Overhead environment: Any dive where you cannot make a direct vertical ascent to the surface. Caves, caverns, wrecks and ice all qualify, and all demand specific training.
Cavern: The zone still within natural daylight, inside defined limits of distance and depth. Cavern training is where overhead instruction usually begins.
Cave: Beyond the reach of daylight. Everything now depends on your lights, your guideline and your gas plan.
Guideline: Continuous line laid from open water into the system, providing a physical route out when visibility is gone. The single most important piece of cave kit.
Silt-out: Loss of visibility from disturbed sediment, often to literally zero. Caused by fin wash, poor trim, or contact with the floor or ceiling.
Halocline: The shimmering boundary where fresh water meets salt water. Visually striking, and disorientating when you swim through it.
Sump: A flooded section of an otherwise dry cave — often the link between two air passages, and a speciality of its own.
Penetration: Linear distance travelled into a system from the entrance. The figure quoted in exploration accounts, and the number your gas plan is really about.
Sidemount: Carrying cylinders on the diver's sides rather than the back, allowing passage through restrictions that a back-mounted set will not fit.

Background reading only

Understanding karst hydrology tells you why a cave floods. It does not tell you how to get out of one. See Safety & Training.