Sinkhole may exhibit a visible opening into a cave below. In the case of exceptionally large sinkholes, such as the Minyé sinkhole in Papua New Guinea or Cedar Sink at Mammoth Cave National Park in Kentucky, an underground stream or river may be visible across its bottom flowing from one side to the other. Sinkholes are common where the rock below the land surface is limestone or other carbonate rock, salt beds, or in other soluble rocks, such as gypsum, that can be dissolved naturally by circulating ground water. Sinkholes also occur in sandstone and quartzite terrains. As the rock dissolves,
Danube Sinkhole other hand, to decide the smoldering controversy over the quantitative impact of the Danube sinkhole, which is known as the Danube Sinkhole case (). As the karst is developing, future sinkhole stretches cannot be accurately predicted. The underground river Danube annually removes about 7,000 tons of lime, that is 2,700 cubic meters, from the karst system. Future expansions or collapses in the underground system are conceivable, which would show above ground as sinkholes or ponors. In the long run, today's upper Danube will probably be redirected entirely into the Radolfzeller Aach, and thus to the Rhine. This would mean that