Karst landscapes and sinkholes

On its way through air and soil, rainwater takes up carbon dioxide and becomes slightly acidic. This weakly acidic water dissolves limestone and dolomite along joints and bedding planes. Gypsum dissolves even more easily. Over long periods, fine cracks widen into tubes and caves through which water flows underground.

A sinkhole forms when the roof of such a cavity collapses or when loose material such as sand and clay trickles down into cracks from above. A depression or a hole then forms at the surface. Some hollows develop slowly over centuries, others collapse suddenly.

Triggers for sudden collapses can be heavy rain, long droughts or human activity, for example when groundwater is pumped out or water leaks from pipes. The US Geological Survey describes these processes in detail in its Water Science School.

Typical signs are funnel-shaped hollows in meadows and forests, called dolines, streams that suddenly vanish into a hole, strong springs at the foot of slopes and bare, furrowed limestone surfaces. Dry valleys without a stream can also point to karst, because the water drains away underground.

Geological maps or information boards often tell you whether you are in a karst area. In Europe there are large karst areas, for example, in the Limestone Alps, the Dinaric mountains and parts of central Germany with gypsum karst. National geological surveys publish maps of areas where sinkholes occur.

The edges of dolines, shafts and fresh collapses can give way, because there are cavities beneath them that you cannot see from above. Stay on paths, do not step to the edge of hollows or shaft openings and never cross barriers. Keep children and dogs close to you. In fog and snow extra care is needed, because openings can be hidden.

Do not investigate a fresh sinkhole, however curious you are. Have it cordoned off and report it to the local authority. Anyone living in a karst area who notices new depressions, cracks in walls or floors, tilting posts or water suddenly draining away near the house should also avoid the area and report it. In immediate danger, call the local emergency number (112 in the EU).

Caves in karst areas can fill with water quickly when it rains, even if it is only a shower outside. So only go into show caves or on guided tours with a caving club, and never when it is raining or a storm warning is in place.

In ordinary soil, water seeps slowly through earth, sand and gravel, and some contaminants are held back on the way. Karst is different: water flows quickly and almost unfiltered underground through cracks and tubes. Whatever gets into the ground on a pasture, by a road or in a village above can come out of a spring a short time later.

That is why clear, cold spring water can still be contaminated. Always treat water from karst springs and streams before drinking, for example by boiling it, and follow the advice of the local authorities. A filter alone does not make water safe.

Sinkholes are rare, and there are often warning signs such as new depressions, cracks or tilting posts. Karst areas are usually well researched and mapped. If you know the signs and keep your distance, you can enjoy these landscapes with their caves, springs and rock formations safely.

This text gives no construction or survey advice and explains no caving technique. Anyone who owns or wants to buy a house in a karst area should turn to experts and the national geological survey with their questions.