In southern Highland County, some landowners watch anxiously as lawns and farm lots slowly sag into the earth, causing birdfeeders, swing sets, clothesline posts, fence posts, and outbuildings to tilt out-of-plumb. In Columbus, city engineers struggle to contain the rapidly escalating cost of a multimillion-dollar tunneling project that is delayed by the unanticipated presence of an ancient buried valley and ancient caverns filled with sticky red clay. In north-central Ohio, rural residents from Bellevue to Castalia are frustrated by a maze of subterranean passageways that allow pesticides, fertilizers, herbicides, and animal wastes to move unabated from fields into underlying aquifers. Children might easily imagine that some rock-eating Stygian monster from Star Wars has been loosed upon the state to wreak such havoc. Geologists, however, quickly recognize that the common factor in all these problems is karst.
 
Karst is a landform that develops (p. 7) on or in limestone, dolomite, or gypsum by dissolution and that is characterized by the presence of features such as sinkholes, underground (or internal) drainage through solution-enlarged fractures (joints), and caves. Approximately 10 percent of Earth's continents, including about 20 percent of the conterminous United States, is underlain by karst-modified bedrock. The greatest concentration of karst terrain and karst features in the nation is in Florida, where 100 percent of the state is underlain by karst-forming carbonate bedrock.
 
Nearly one-third of Ohio is directly underlain by Ordovician, Silurian, and Devonian carbonate bedrock that has the potential to have developed some karst features during its existence. This potential is confirmed by the fact that solution-enlarged fractures are visible in the highwalls of most of the state's limestone and dolomite quarries. Even so, less than 2 percent of the Ohio landscape includes karst terrain. The percentage is low because most near-surface carbonate bedrock in Ohio is covered with a thick mantle of glacial deposits which greatly impede or preclude active karst-forming processes. In addition, the abrasive work of Ice Age glaciers is believed to have destroyed much of the karst terrain that had developed in Ohio prior to glaciation.
 
Karst, like landslides and coastal erosion, is a geologic hazard. Sudden collapse of an underground cavern or opening of a sinkhole can cause surface subsidence that can severely damage or destroy any overlying structure such as a building, bridge, or highway. Improperly backfilled sinkholes are prone to both gradual and sudden subsidence and similarly threaten overlying structures. Sewage, animal wastes, and agricultural, industrial, and ice-control chemicals entering sinkholes as surface drainage are conducted directly and quickly into the ground-water system, thereby posing a severe threat to potable water supplies. Because of such risks, many of the nation's state geological surveys and the U.S. Geological Survey are actively mapping and characterizing the nation's karst regions.
 
The Ohio Geological Survey recently released Digital Chart and Map Series (DCMS) 24, Known and probable karst in Ohio, a color, plot-on-demand, 1:500,000-scale digital map that depicts areas of probable karst and locations of documented karst features (DCMS 24 has been superceded by Digital Map Series EG-1, 2002). Plotted on a county/township base, the map includes generalized bedrock-geology information for areas of probable karst.
 

Large sinkhole developed in Peeble-Lilley Dolomites in eastern Oliver Township, Adams County. Note dog at center of depression. Photo by C. Scott Brockman, 1997.

Although DCMS 24 is the first statewide map showing known and probable karst areas of the state, reports on Ohio karst features have been published for more than a century. The first statewide survey of Ohio caves was conducted in 1924 by Ohio State University graduate student George W. White, working under the direction of State Geologist John A. Bownocker. White's cave survey included maps and descriptions of more than 20 of the state's major caves and was published in a 1926 Ohio Journal of Science article entitled "The limestone caves and caverns of Ohio." Following this investigation, White embarked on a distinguished career in geology that would span seven decades, earn him international recognition for his pioneering efforts in the field of glacial stratigraphy, and include an appointment as Ohio's seventh State Geologist (1946-47).
 
Following White's work in 1924, little if anything was done to document Ohio karst features on a statewide basis until 1952, when the Ohio Geological Survey and the Central Ohio Grotto of the National Speleological Society collaborated to establish a comprehensive survey of Ohio caves intended to include: (1) cave locations, (2) descriptions of caves, and (3) discussions of geological features associated with caves. P. M. Smith of the Central Ohio Grotto published a brief outline of these objectives in a 1953 Ohio Journal of Science article but never published any of the data collected by the cave survey. In 1974, Warren Luther of the Central Ohio Grotto compiled 22 years of Ohio Cave Survey data; however, this compilation also was never published. In 1978, responsibility for the Ohio Cave Survey was assumed by the Wittenberg University Speleological Society under the direction of Professor Horton H. Hobbs, III. Descriptions of caves in the Ohio Cave Survey have been published regularly since 1978 in the Wittenberg University Speleological Society's journal Pholeos.
 
The lack of statewide karst terrain information became a major issue in September 1993, when an Ohio Blue Ribbon Commission released its Recommendations on siting criteria and development requirements for a regional low-level radioactive waste disposal facility in Ohio. Ground-water flow is very rapid in the solution-enlarged bedrock fractures underlying karst terrain, thus any radioactive contaminants entering such fractures through sinkholes or other karst features would be dispersed quickly, thereby contaminating ground water over a broad area in short order. Knowing this, the Blue Ribbon Commission recommended that, "The [Ohio low-level radioactive-waste disposal] site shall not be located above a bedrock formation likely to cause [sic] karst features to develop on the site." The Ohio General Assembly subsequently enacted Section 3747.12 of the Ohio Revised Code, which states, "The disposal site shall not be located in areas of known or probable karst. As used in division (A)(13) of this section, 'karst' means a terrain with an assemblage of landforms such as sinkholes and caves that are due to solution weathering of predominantly carbonate bedrock."
 
Because much of western Ohio is underlain by carbonate strata susceptible to the development of karst, the Research and Technology Committee of the Ohio Low-Level Radioactive Waste Disposal Facility Development Authority determined that karst terrain should be considered an exclusionary criterion in the statewide screening process to identify areas of the state that obviously would be unsuitable for disposal of low-level radioactive waste and that should receive no further consideration in a site-selection process. The Committee then asked the Division of Geological Survey to prepare a proposal for preparation of a 1:500,000-scale map of known and probable karst that could be used in the statewide screening process for a low-level radioactive-waste disposal facility.
 
A proposal for a six-month-long karst mapping project was submitted to the Committee and approved in April 1997. Karst mapping began immediately thereafter, and a 1:500,000-scale digital map of Ohio showing locations of known karst features and areas of probable karst was completed on schedule in October 1997. However, the map was never delivered to the Ohio Low-Level Radioactive Waste Facility Development Authority, as that agency closed its doors on August 31, 1997, following a decision by the Midwest Low-Level Radioactive Waste Compact to cease all facility development operations in Ohio.
 
In recognition that karst is a significant geologic hazard in Ohio, and that many Ohioans have an interest in knowing the location of the state's principal karst regions, the Division decided at the conclusion of the six-month project to produce a new state map of known and probable karst for public release. Accordingly, the digital cartographic files of the locations of known and probable karst were merged with digital bedrock-geology maps to produce the new open-file map, DCMS 24, Known and probable karst in Ohio. This new map, prepared by Richard R. Pavey, Dennis N. Hull, C. Scott Brockman, Gregory A. Schumacher, David A. Stith, E. Mac Swinford, Terry L. Sole, Kim E. Vorbau, Kevin D. Kallini, Emily E. Evans, Ernie R. Slucher, and Robert G. Van Horn, is designed to provide an overview of Ohio karst and is not intended for site-specific investigations. The five most significant Ohio karst regions appearing on the new map are described below.