Groundwater can dissolve rock. This fact is key to understanding how caverns and sinkholes form. Soluble rocks, especially limestone, underlie millions of square kilometers of Earth’s surface, and in these rocks, groundwater carries on its important role as an erosional agent (Figure 1). Limestone is nearly insoluble in pure water but is quite easily dissolved by water containing small quantities of carbonic acid, and most groundwater contains this acid. It forms because rainwater readily dissolves carbon dioxide from the air and from decaying plants. Therefore, when groundwater comes in contact with limestone, the carbonic acid reacts with calcite (calcium carbonate) in the rocks to form calcium bicarbonate, a soluble material that is then carried away in solution.

Caverns

The most spectacular results of groundwater’s erosional handiwork are limestone caverns. In the United States alone, about 17,000 caves have been discovered. Although most are relatively small, some have spectacular dimensions. Carlsbad Caverns in southeastern New Mexico and Mammoth Cave in Kentucky are famous examples. One chamber in Carlsbad Caverns has an area equivalent to 14 football fields and enough height to accommodate the U.S. Capitol Building. At Mammoth Cave, the total length of interconnected caverns extends for more than 685 kilometers (426 miles).

Most caverns form at or below the water table, in the zone of saturation. Here acidic groundwater follows lines of weakness in the rock, such as joints and bedding planes. As time passes, the dissolving process slowly creates cavities and gradually enlarges them into caverns. Material removed by the groundwater is eventually discharged into streams and carried to the ocean.

The features that arouse the greatest curiosity for most cavern visitors are the stone formations that give some caverns a wonderland appearance. These are not erosional features, like the caverns in which they reside, but depositional features. They are created by the seemingly endless dripping of water over great spans of time. The calcium carbonate that is left behind produces the limestone we call travertine. These cave deposits, however, are also commonly called dripstone, an obvious reference to their mode of origin.

Although the formation of caverns takes place in the zone of saturation, the deposition of dripstone is not possible until the caverns are above the water table, in the unsaturated zone. This commonly occurs as nearby streams cut their valleys deeper, lowering the water table as the elevation of the rivers drops. As soon as the chamber is filled with air, the conditions are right for the decoration phase of cavern building to begin.

Of the various dripstone features found in caverns, perhaps the most familiar are stalactites. These icicle-like pendants hang from the ceiling of a cavern and form where water seeps through cracks above. When water reaches air in the cave, some of the dissolved carbon dioxide escapes from the drop, and calcite begins to precipitate. Deposition occurs as a ring around the edge of the water drop. As drop after drop follows, each leaves an infinitesimal trace of calcite behind, and a hollow limestone tube is created. Water then moves through the tube, remains suspended momentarily at the end, contributes a tiny ring of calcite, and falls to the cavern floor. The stalactite just described is appropriately called a soda straw (Figure 2A). Often the hollow tube of the soda straw becomes plugged, or its supply of water increases. In either case, the water is forced to flow and deposit along the outside of the tube. As deposition continues, the stalactite takes on the more common conical shape.

Formations that develop on the floor of a cavern and reach upward toward the ceiling are called stalagmites (Figure 2B). The water supplying the calcite for stalagmite growth falls from the ceiling and splatters over the surface. As a result, stalagmites do not have a central tube and are usually more massive in appearance and more rounded on their upper ends than stalactites. Given enough time, a downward-growing stalactite and an upward-growing stalagmite may join to form a column.

Karst Topography

Many areas of the world have landscapes that, to a large extent, have been shaped by the dissolving power of groundwater. Such areas are said to exhibit karst topography, named for the Krs region in the border area between Slovenia and Italy, where such topography is strikingly developed. In the United States, karst landscapes occur in many areas that are underlain by limestone, including portions of Kentucky, Tennessee, Alabama, southern Indiana, and central and northern Florida (Figure 3). Generally, arid and semiarid areas do not develop karst topography because there is insufficient groundwater. When karst features exist in such regions, they are likely to be remnants of a time when rainier conditions prevailed.

Sinkholes

Karst areas typically have irregular terrain punctuated with many depressions called sinkhole or, simply, sinks (Figure 4). In the limestone areas of Florida, Kentucky, and southern Indiana, literally tens of thousands of these depressions vary in depth from just 1 to 2 meters (3 to 6 feet) to a maximum of more than 50 meters (165 feet).

In addition to a surface pockmarked by sinkholes, karst regions characteristically show a striking lack of surface drainage (streams). Following a rainfall, runoff is quickly funneled below ground, through sinks. It then flows through caverns until it finally reaches the water table. Where streams exist at the surface, their paths are usually short. The names of such streams often give a clue to their fate. The Mammoth Cave area of Kentucky, for example, is home to Sinking Creek, Little Sinking Creek, and Sinking Branch. Some sinkholes become plugged with clay and debris, creating small lakes or ponds.

Tower Karst Landscapes

Some regions of karst development exhibit landscapes that look very different from the sinkhole-studded terrain depicted in Figure 3 and Figure 4. One striking example is an extensive region in southern China that is described as exhibiting tower karst. As Figure 6 shows, the term tower is appropriate because the landscape consists of a maze of isolated steep-sided hills that rise abruptly from the ground. Each is riddled with interconnected caves and passageways. This type of karst topography forms in wet tropical and subtropical regions having thick beds of highly jointed limestone. In such settings, groundwater dissolves large volumes of limestone, leaving only these residual towers. The development of a karst landscape occurs more rapidly in tropical climates due to the abundant rainfall and greater availability of carbon dioxide from the decay of lush tropical vegetation. The extra carbon dioxide in the soil means there is more carbonic acid for dissolving limestone. For these reasons, weathering occurs rapidly, eliminating caves and underground voids, and what is left behind are towers of solid limestone. Other tropical areas of tower karst development include portions of Puerto Rico, western Cuba, and northern Vietnam.

• Groundwater dissolves rock, in particular limestone, leaving behind void spaces in the rock. Caverns form at the zone of saturation, but later, dropping of the water table may leave them open and dry—and available for people to explore.

• Dripstone is rock deposited by dripping of water containing dissolved calcium carbonate inside caverns. Features made of dripstone include stalactites, stalagmites, and columns.

• Karst topography develops in limestone regions and exhibits irregular terrain punctuated with many depressions called sinkholes. Some sinkholes form when the cavern roofs collapse.

• caverns: Large, underground chambers formed by the weathering of rock.

• karst topography: Landscapes produced from the dissolution of underground rock, such as limestone, characterized by sinkholes, sinking streams, and caverns.

• sinkhole (a.k.a. sink): Depression in the ground surface caused by the collapse of surface materials due to underground erosion. Also called a sink.

• stalactites: Mineral or rock deposits that hang from the ceiling of a cavern.

• stalagmite: Formations that rise from the floor of a cave, produced by the precipitation of minerals from dripping water.