The sinkhole starts as a depression in the land surface which is typically the result of dissolving bedrock beneath the surface, beginning a process that creates instability in the soil and rock above. Sinkholes can occur singly, or in groups in close proximity to one another. Sinkholes range from a few feet to hundreds of feet in depth. Not all topographical depressions are sinkholes. Sinkholes are common where the rock below the land surface is limestone, carbonate rock, salt beds, or rocks that can naturally be dissolved by ground water circulating through them. As the rock dissolves, spaces and caverns develop underground. Sinkholes are dramatic because the land usually stays intact for a while until the underground spaces just get too big. If there is not enough support for the land above the spaces then a sudden collapse of the land surface can occur. These collapses can be small or they can be huge and can occur where a house or road is on top.