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,
DNS sinkhole DNS sinkhole A DNS sinkhole, also known as a sinkhole server, Internet sinkhole, or Blackhole DNS is a DNS server that gives out false information, to prevent the use of a domain name. A sinkhole is a DNS provider that supplies systems looking for DNS information with false results, allowing an attacker to redirect a system to a potentially malicious destination. DNS sinkholes have also historically been used for non-malicious purposes. When a computer visits a DNS source to resolve a domain name, the provider will give a result if possible, and if not, it will send the resolution system