How Caves Form

A hidden world beneath the ground

Deep below hills and mountains, there are hidden chambers, tunnels and tall stone "icicles" that glitter in torchlight. These are caves — natural empty spaces inside rock, large enough for a person to enter. Some caves are short and shallow, while others form vast underground systems stretching for hundreds of kilometres.

But where do caves come from? They are not dug by people, and they did not always exist. Most caves are slowly carved out by water, rock and time. In this lesson we will explore the main ways caves form, look at the dazzling shapes that grow inside them, and discover why these underground worlds matter.

Solution caves: dissolved by acid water

The most common large caves are solution caves, and they are usually found in a rock called limestone. To understand them, we need to know a little chemistry.

When rain falls, it passes through the air and then soaks into the soil. Along the way, the water absorbs carbon dioxide gas. This makes the water slightly acidic — it becomes a very weak acid called carbonic acid. It is far too weak to hurt you, but it is strong enough to do something remarkable.

Limestone is mostly made of a mineral called calcite (calcium carbonate), and this mineral slowly dissolves in weak acid. So as the acidic water trickles down through cracks in the limestone, it dissolves tiny amounts of rock and carries them away. Drip by drip, year after year, the cracks widen into passages, and the passages grow into chambers. Over thousands or even millions of years, an entire cave system can be hollowed out.

This is a kind of chemical weathering. Landscapes full of such caves, sinkholes and underground rivers are called karst landscapes. Famous examples include the Mammoth Cave system in the United States, the longest known cave on Earth, and the spectacular caves of Guilin in China.

Stalactites, stalagmites and stone curtains

Once a cave exists, water keeps dripping through it — and this water builds beautiful shapes called speleothems.

Here is how it works. Water seeping through limestone dissolves calcite as it travels. When a drop reaches the open air of a cave, some of the carbon dioxide escapes from the water (a bit like fizz leaving a fizzy drink). This makes the water unable to hold as much dissolved calcite, so a tiny ring of calcite is left behind.

• Stalactites grow down from the ceiling. Each drop hanging from the roof leaves a microscopic ring of calcite. Over time these rings stack into a stone "icicle" pointing downward.
• Stalagmites grow up from the floor. When the drop finally falls and splashes, it leaves calcite on the ground, building a mound or pillar that points upward.
• If a stalactite and a stalagmite meet, they join into a column.
• Thin sheets of dripping water can leave flowstone and rippling stone "curtains".

These features grow astonishingly slowly, sometimes less than a millimetre a year. A stalactite the length of your arm may have taken many thousands of years to form. That is why cavers are told never to touch or break them — a moment's damage destroys something older than human history.

Other ways caves form

Not all caves are dissolved out of limestone. Nature has several other ways of making them.

Sea caves form along coasts. Powerful ocean waves crash against cliffs again and again. Where the rock has cracks or soft, weak spots, the waves wear it away faster, eventually carving out a cave at the base of the cliff. This is erosion by water, and it links to the restless power of the ocean you can read about in oceans and sea life. Fingal's Cave in Scotland is a famous sea cave.

Lava tubes form during volcanic eruptions. As a river of hot lava flows, its outer surface cools and hardens into a crust while the lava inside keeps flowing. When the eruption stops and the last lava drains away, it leaves behind a long, hollow tunnel. Lava tubes are common in Hawaii and Iceland. They are connected to the same fiery forces described in volcanoes and earthquakes.

Glacier caves and ice caves form when meltwater or warm air carves tunnels inside or beneath a glacier. Because ice melts far faster than rock, these caves can change shape from year to year.

Life in the dark

Caves may seem empty and lifeless, but many are surprisingly full of life. Because there is no sunlight deep inside, no plants can grow there, so the food chain works differently from the surface.

• Bats are the most famous cave animals. They roost in caves by day and fly out at night to hunt insects. Their droppings, called guano, feed many other cave creatures.
• Blind cave fish and pale, eyeless cave insects have adapted to total darkness. Many have lost their eyes and colour entirely, because they are no use where there is no light.
• Tiny bacteria and fungi form the base of the cave food web, feeding on guano and anything washed in from outside.

These animals show how living things can adapt to even the most extreme places.

Why caves matter

Caves are valuable for many reasons. They store and filter huge amounts of groundwater, which people drink. They are important habitats for rare species like bats, many of which are now endangered. They also act as natural museums: scientists study ancient stalactites and cave layers to learn about past climates, and some caves contain prehistoric paintings made by humans tens of thousands of years ago, such as the cave art of Lascaux in France.

Because caves form so slowly and are so fragile, they are easily damaged by pollution, litter and careless visitors. Protecting them protects a record of Earth's deep past.

Try it yourself: grow your own stalactite

You can model how dripping water builds cave formations using everyday materials. Ask an adult for help and be patient — like a real cave, it takes time.

1. Fill two cups or jars with warm water. Stir in as much washing soda (or Epsom salts) as will dissolve in each, until no more will dissolve.
2. Place the two jars a short distance apart, with an empty plate or saucer between them.
3. Twist a length of cotton wool or thick wool yarn into a string. Put one end into each jar so the middle of the string dips down low over the plate.
4. Leave it somewhere safe for about a week and watch.

The mineral solution will slowly soak along the string and drip onto the plate. As the water evaporates, it leaves the mineral behind, growing a small stalactite along the string and a stalagmite rising from the plate below — exactly the process that builds these features in real caves, only millions of times faster.