Hypothetical section of a karst area exhibiting the different levels at which water flows and showing surface morphologies (taken from Muscio)

The term "karst area" refers to a terrain characterised by a peculiar water regimen. Under normal conditions, rainwater percolates and flows underground and no stream channels appear above ground. The surface-draining pattern becomes active only during floods.

On the one hand, Karst areas offer no classic rivers or stream valleys; however, they do offer other landforms. These include: closed depressions called dolines, and heavily cracked rocks (showing cracked fields or grooves). Underground, an elaborate network of water-collecting channels often creates wide galleries and chambers, some of which may connect to the surface of the ground, thus giving shape to caves and abysses.

The karst phenomenon can only develop in the presence of a specific rock pattern, that of carbonate rocks (e.g.: limestone). This rock is normally resistant to erosion by water but be comes non-resistant to erosion when it contains a given percentage of carbon dioxide, a gas which is available in the air and, above all, in the ground.

Rainwater, enriched with carbon dioxide, very slowly "dissolves" calcium carbonate, enlarges the cracks in the rocks and sinks deep. As water continues to widen the cracks, wider and wider underground galleries start to form. Galleries eventually become accessible to man. Rainwater carries the calcium carbonate it has dissolved along its path into these hollow spaces and forms the concretions that typically adorn natural caves.

The inside of the Grotta di Santa Maria

It follows that all caves are run through by water (active caves) or have been in the past (fossil caves).
As water sinks deeper and deeper, flow levels change. However, this simplification is not al ways applicable due to the complexity of geological occurrences that normally influence the evolution of areas. Water is therefore a key element in the development of the karst phenomenon and natural caves often provide important water reservoirs so much so that many waterworks, including in this area, rely on karst aquifers. Karst aquifers, however, are particularly sensitive to pollution and this is why effective programmes need to be implemented for their protection.