Caves & karst

Deep below the surface there are sometimes huge halls, silent lakes and passages that run for kilometres through the rock. Nobody dug them, and yet they are there. The builder is ordinary rainwater, which dissolves rock over very long times, drop by drop. Where this happens on a grand scale, a very special landscape forms, called karst.

Water that dissolves stone

Pure water can hardly harm limestone. But rainwater takes up carbon dioxide from the air and above all from the soil, and this forms carbonic acid, the same very weak acid that is also in sparkling water. Limestone consists of calcium carbonate, often the remains of ancient sea creatures, and this is exactly the substance the slightly acidic water can slowly dissolve. It seeps into fine cracks, dissolves rock from the walls and makes the cracks a tiny bit wider as it goes. Over tens of thousands of years hairline cracks become fissures, fissures become passages, and passages become whole cave systems in which underground streams flow.

Dripstones: icicles made of lime

In a finished cave the story runs backwards. When lime-rich water drips from the ceiling, it releases part of its carbon dioxide into the cave air and can no longer hold the dissolved lime. A wafer-thin trace stays behind on the ceiling, and drop by drop a hanging icicle grows, a stalactite. Where the drops land below, a stalagmite grows up from the floor to meet it. This happens unimaginably slowly, often by only about 0.1 millimetres per year. If the two meet after many thousands of years, they join into a column.

Where many hollow spaces form beneath a landscape, sooner or later it shows at the surface. When the roof of a cave collapses, a funnel-shaped hollow appears, a sinkhole. Streams suddenly vanish into a crack and continue underground until they reappear somewhere else as a strong spring. Such a hole-riddled limestone landscape is called karst, named after a region in Slovenia where it can be seen especially well. The longest known cave system in the world is Mammoth Cave in the USA, with more than 600 kilometres of surveyed passages.

Karst also concerns us very directly. In many regions of the world drinking water comes from karst springs, because the hole-riddled limestone acts like a giant water store. At the same time this water is delicate: since it is barely filtered on its way through the wide fissures, dirt from the surface quickly reaches the depths. So if you visit a dripstone cave, you are not only standing in a natural wonder but right inside the plumbing system of an entire landscape.