Natural Caves

Spelunk Deep Into This Article About Speleology

Spelunking, the exploration of caves, is adventurous activity. Getty Images photo by Alan Cressler

It’s a Wilderness Down There

Caves are empty underground spaces, mysterious pits of darkness calling out for exploration. They can have many parents among geologic processes. Mineral dissolution, volcanic eruptions, tectonic movement, and erosion from water or wind are some of the ways caves are born. Mammoth Cave in Kentucky is the world’s longest surveyed cave, with 365 miles (587.41 km) of passages and new branches waiting to be explored. Borneo’s Sarawak Cave has the largest single chamber, where an airline could comfortably park its fleet of 747s. Krubera Cave, the world’s deepest explored cave, lies in Georgia (the country, not the state), and plunges 7,208 feet (2,197 m) into the depths of the Earth.

Myths and Monsters

Caves not only dot all seven continents and Oceana, but also strongly inhabit the human imagination. Myths and sagas and literature and songs are full of tales involving caves. Sometimes caves can be protective. For example, many Greek gods were supposedly born and sheltered in caves. Romulus and Remos, the abandoned baby twins who legend says grew up to found Rome, were said to have been suckled by a she-wolf in a cave on the Palatine Hill. 

More often, though, the caves found in myths and stories are threatening and terrifying, home to monsters and dragons and thieves. Heroes often battle villains and ogres in the recesses of their caves: In Homer’s Odyssey, Odysseus blinds the fearsome cyclops Polyphemus in the cave where he has been feasting on Odysseus’s crew members, for example; in a famed Nordic saga, Beowulf battles the monster Grendel and his mother in their cave.

Stories often tell of great treasure hidden in caves. Think about Ali Baba and the Forty Thieves from One Thousand and One Arabian Nights, or Tom Sawyer and Huckleberry Finn finding gold in McDougal’s Cave. (Great treasure is found in real caves, too; in 1947 a Bedouin shepherd discovered the priceless Dead Sea Scrolls hidden in a cave for millennia.)

It’s All in the Timing

Caves often are classified by when they were formed in relation to the rock that surrounds them.  

Primary caves—such as lava tubes—form at the same time as the rock around them. Lava tubes are the primary example of primary caves. When a crust forms over streams of lava flowing from a volcano, hot lava continues to flow beneath it until the eruption subsides. The lava drains, leaving behind empty tunnels called lava tubes below the hardened crust. 

Secondary caves—the most common type—result from water (and sometimes wind) relentlessly eroding rocks over millions of years. Most of these caves form in landscapes called karst, which are made up soluble rocks, especially limestone, but also others such as gypsum, dolomite, marble, and even salt. What happens? Rain and groundwater containing weak natural acids seep through the ground and slowly dissolve calcite, the main mineral in karst rocks. 

Tertiary caves—dangerous to explore because they are often unstable—form when boulders tumble down a mountain or crumble from a cliff, sometimes as the result of earthquakes. Empty chambers that sometimes form within those jumbles of rocks are called talus caves. Tertiary caves also result when an existing cave collapses. 

Water Is the Culprit 

Karst caves are called solutional caves because a solution of acid and water creates them. But karst caves aren’t nature’s only way of sculpting caves with water. 

Sea caves form at the base of cliffs, carved from rock by relentless erosion from waves. One of the world’s most famous sea caves is the celebrated Grotto Azziera (Blue Grotto), a major tourist attraction on Italy’s isle of Capri. Sunlight from the entrance reflects on the water in the cave and fills the semi-flooded cavern with radiant blue light.

Meltwater coursing toward the sea beneath glaciers can leave behind glacier caves, which are not the same as ice caves, another member of the solutional cave family. Ice caves occur in cold climates and they appear to be made of ice, but they are really rock caves in which ice never melts. Austria is home to the world’s largest ice cave, Eisriesenwelt Cave, which stretches for nearly 50 km., or 30 miles.

Stalactites vs. Stalagmites

Nature is an accomplished interior decorator, filling caves with amazing formations of mineral deposits. Most people who are interested in geography have heard of stalactites and stalagmites, but it’s hard to keep them straight. Which one is which?

Both are pointy mineral deposits that result when water dissolves rock, especially very soluble limestone. (The Greek root word is “stalagmias,” meaning to drip.)  Here’s the difference: Icicle-like stalactites, formed by the dripping minerals, hang from a cave’s roof, while stalagmites rise where the mineral-laden water drips onto the floor below. (Sometimes stalactites and stalagmites meet in the middle, forming columns.)

Here’s a neat trick (it’s called mnemonics) so you’ll never get stalactites and stalagmites confused again. Just remember that the “ct” in “stalactites” stands for “ceiling tears,” and the “gm” in “stalagmites” stands for “ground mounds”.

Stalactites and stalagmites are the most familiar cave sculptures, but hardly the only ones. Sometimes water cascades down a cave wall, leaving behind rippled sheets of calcite called flowstones. Other deposits turn into fanciful shapes resembling pipe organs or wedding cakes.

Cave Critters

The common denominator within caves is enveloping darkness. That appeals to nocturnal creatures such as bats, which sleep in caves during the day and emerge at dusk to hunt insects. (Every day as the sun is setting, tourists gather outside Carlsbad Caverns in New Mexico to watch bats explode from the mouth of the cave and darken the sky.) 

The darkness has also allowed interesting cave-dwelling creatures—fish, salamanders, worms, insects—to evolve with special accommodations to live in a world without light. For example, blind fish (who needs eyes in the pitch darkness, anyway?) have developed other heightened senses to compensate for lack of light. Many cave-dwelling critters have lost pigment and turned white, and some of them are actually transparent. 

Other examples: Once upon a time, salamander-like cave-dwelling olms were believed to be baby dragons. In New Zealand, millions of tiny glow worms illuminate the famous Waitomo Caves. In short, unique animal species all over the world have made caves their homes sweet homes.

And don’t forget that people are cave-dwellers too. There is archeological evidence that the earliest humanoids not only inhabited caves, but also used them for special purposes such as burials and religious rites.

Caves as Canvas

The very first artwork created by humans appeared on the walls of caves all over the world. Usually the paintings, as much as 35,000 years old, depict animals ranging from mammoths to bears, wolves, bulls, and more.

Another common motif among cave paintings is hand stencils. Anthropologists don’t really know the purpose of this ancient cave art, but speculate that it could have had religious connotations, or been used to communicate with other hunter-gatherers, or recorded hunting triumphs. (Maybe, though, those artists were simply prehistoric Michelangelos!)

Some of the most famous cave paintings have been discovered in France (Lascaux, for example), Spain (Alramira, for example), and throughout Australia and Southeast Asia.

Exploring Caves

Speleology is the study of caves, and spelunking is the actual process of physically exploring them. Scientists and other cave enthusiasts in the U.S. belong to the National Speleological Association (, whose members promote safe cave exploration and cave conservation.