What if the ground were made of limestone?

Limestone is a kind of rock. Much of it was made from the remains of sea creatures, long ago.

Rainwater can slowly dissolve it. What if all the ground were limestone?

Limestone is a rock that formed long ago, much of it from the remains of sea creatures piled up and pressed together. It has a surprising weakness: rainwater can slowly dissolve it.

What if all the ground under our feet were limestone? In this thought experiment, you can follow three paths: the caves that water carves underground, the sinkholes that can open at the surface, and the springs where underground rivers come back into daylight.

The National Park Service classes limestone as a biologic sedimentary rock: material from marine organisms that accumulated and was compacted and cemented. It dissolves in weakly acidic water, so limestone landscapes develop a distinctive set of features known as karst.

This thought experiment imagines limestone everywhere. It asks where rain would go, what would open up underground, and how people would get their water. Real karst regions, which USGS says cover about 20 percent of the US land surface, provide the evidence; a planet of nothing but limestone is reasoning, labeled as ideas.

Rain soaks into the soil. There it picks up a gas called carbon dioxide and becomes a very weak acid.

The weak acid slowly dissolves limestone and makes cracks bigger and bigger.

Rainwater is slightly acidic. As it soaks through soil, it picks up even more carbon dioxide, the same gas that makes soda fizzy, and forms a very weak acid called carbonic acid.

When this water reaches limestone, it seeps into cracks and slowly dissolves the rock. Small cracks become wider, then become tunnels. Underground streams flow through the tunnels and make them bigger. Over a very long time, a whole cave system can form.

NPS explains that rainwater mixing with soil carbon dioxide forms carbonic acid, the weak acid in carbonated drinks. Though weak, it dissolves limestone over long periods. The acidified water percolates down until it finds a way into the rock, usually through joints, fractures, faults or bedding planes.

Once a pathway opens, flow increases, bringing in more acidic water and speeding growth: a positive feedback. Streams in caves abrade and scour passages with the sediment they carry. Dolomite and marble dissolve in the same way, and gypsum and halite dissolve even more easily. A different route, sulfuric acid, carved the limestone caves of Carlsbad Caverns.

Let's pretend all the ground is limestone.

Rain might sink into the ground instead of running into streams. There might be caves under many places. This is only pretend.

Now imagine all the ground were limestone. When it rained, much of the water might sink through cracks and sinkholes instead of running off into streams. Rivers might disappear underground and pop back up as springs somewhere else.

Caves might stretch under towns, farms and forests. Builders might need to check carefully for hollow spaces before putting up houses or roads. And people might get most of their water from underground. These are ideas, worked out from limestone areas that exist today.

Real karst suggests the picture. Surface drainage might be sparse, with streams sinking into the ground and reemerging at springs, since sinkholes by definition have no external surface drainage. Groundwater might flow fast through connected conduits rather than slowly through soil.

Water supply might depend heavily on karst aquifers, which USGS describes as highly productive but extremely vulnerable to contamination: pollutants at the surface could travel quickly through open conduits. Engineers might need to survey for voids before every road or building. Every one of these outcomes is reasoning from present karst, not a model result.

Inside a cave, water drips from the ceiling. Each drip leaves a tiny bit of rock behind.

Slowly, stone icicles grow. They are called stalactites.

Water does not only dissolve limestone; it can also put it back. When a drop of water hangs from a cave ceiling, it loses some carbon dioxide and leaves a thin ring of mineral. Drop after drop, a hollow tube called a soda straw grows, and it can become a stalactite. Water dripping to the floor builds stalagmites.

At the surface, dissolving rock can leave the ground with nothing underneath to hold it up, forming a sinkhole. Where an underground stream reaches the surface again, it flows out as a spring.

NPS describes stalactites as beginning as hollow soda straws: a drop on the ceiling loses carbon dioxide, a thin film of carbonate precipitates, and successive drops extend the tube. Stalagmites grow from the floor beneath. Cave Minerals of the World lists 38 types of speleothems.

USGS distinguishes dissolution sinkholes, where a depression forms gradually as surface rock dissolves; cover-subsidence sinkholes, which develop gradually in sandy cover; and cover-collapse sinkholes, which can form within hours where clay-rich cover bridges a cavity until it fails. Springs mark where conduits deliver groundwater back to the surface; USGS notes that dissolving water that meets a layer it cannot dissolve, such as sandstone or shale, starts to cut sideways.

Mammoth Cave in Kentucky is the longest cave system we know. Its tunnels stretch over 400 miles!

Some big springs flow out of limestone, too.

USGS says about 20 percent of the land in the United States is karst, the landscape that forms on rocks like limestone. Mammoth Cave in Kentucky, the world's longest known cave system, has more than 400 miles of passages. Florida is famous for its springs.

Karst is also important for drinking water: about 40 percent of the groundwater people drink in the United States comes from karst aquifers. Because water moves quickly through them, they need protecting from pollution.

USGS estimates that karst covers about 20 percent of the US land surface and that about 40 percent of the groundwater used for drinking comes from karst aquifers. Large karst areas also occur in China, Europe, the Caribbean and Australia; Shilin, the Stone Forest near Kunming, China, is a celebrated example of weathered limestone towers.

Sinkhole damage is most common in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania. Human activity can trigger collapse: in January 2010, more than 110 sinkholes formed near Dover, Florida, after heavy groundwater pumping to protect crops from frost lowered the water table. In Missouri's Ozarks, the largest springs flow from limestone and dolomite.