Caves
Caves are natural hollow spaces deep inside the earth, and they are usually incredibly dark. But can life actually survive in a place where the sun never shines?
Who Lives in The Dark?
Yes, they certainly can! Deep in caves where the sun never reaches, specialised creatures like blind cave fish thrive. Because there is no light, these fish have lost their pigment and even their eyes over millions of years of evolution, relying completely on their senses of smell and hearing.
Energy is brought down from the surface! Bats fly out at night to eat insects, then return to the cave to sleep. Their droppings, called guano, land on the cave floor and become fuel for bacteria and fungi, starting an entire underground food web.
Put Your Instincts to the Test
Think about what you might already know about caves and the world underground. Pick an answer for each question, then see if your instincts were right.
About the same as it is in summer. The temperature inside a deep cave stays about the same all year round. Because there are often hundreds of metres of dirt and rock between the cave and the surface, the earth acts as a massive insulator.
The water acts like a mild acid, dissolving the rocks slowly. When rain falls, it picks up carbon dioxide and becomes slightly acidic. As it flows through chalky limestone, it slowly dissolves the rock away drop by drop over hundreds of thousands of years.
A stalagmite. As Bill Nye the Science Guy explains, a formation growing up from the bottom is called a stalagmite, because it might reach the roof. The ones that hold tight to the ceiling are stalactites.
Understanding the Science
Tap each card to reveal the fascinating science of subterranean ecosystems and geology.
Key Concepts
Cave Formation
Tap to learn moreCaves are natural hollow spaces in the earth. They form wherever material like rock or ice gets carried away from an underground space, leaving a gap stiff enough to support a roof. Bill Nye the Science Guy notes that caves can be found almost anywhere, including under water and inside glaciers.
Limestone Dissolving
Tap to learn moreMany common caves are formed in limestone, which is a chalky rock made of ancient seashells. When rainwater soaks into the ground, it acts like a mild acid. Over hundreds of thousands of years, this water flows like an underground river, slowly dissolving the limestone and carving out massive caverns.
Speleothems
Tap to learn moreAs water drips into a cave from the surface, it carries dissolved minerals with it. When a drop of water falls, it leaves a tiny bit of those minerals behind. Over millions of years, these mineral deposits build up to form beautiful rock structures called speleothems. Bill Nye points out that this process happens incredibly slowly, one little drip at a time.
Stalactites and Stalagmites
Tap to learn moreThere is a clever way to remember the difference between these two famous cave formations. A stalactite grows down from the top and holds tight to the ceiling. A stalagmite grows up from the bottom, and it might eventually reach the roof. If they meet, they form a solid column.
Perfect Insulation
Tap to learn moreCaves are highly stable environments. Because there are often hundreds of metres of dirt and earth acting as a roof, caves are insulated from the outside weather. This means the temperature and humidity stay about the same all year round, making caves an incredibly comfortable place for animals to hibernate during the winter.
Trogloxenes (Cave Guests)
Tap to learn moreThe word trogloxene comes from old Greek words meaning cave guest or cave outsider. These are animals like bats and bears. They spend time inside the cave to sleep or hibernate, but they must go outside to the surface to find their food and energy.
Troglophiles (Cave Lovers)
Tap to learn moreTroglophiles are creatures that translate to cave lovers. Animals like earthworms fit into this category. They love the deep, dark, wet environment that a cave provides and can live their entire lives comfortably underground, but they are also capable of surviving in suitable environments above ground.
Troglodytes (Cave Dwellers)
Tap to learn moreTroglodytes are the true cave dwellers, such as blind cave fish and specialised crustaceans. They can absolutely not live anywhere else. Because they live in total darkness, millions of years of evolution have left them with no skin pigment and often no eyes at all. Instead, they survive using highly developed senses of smell and touch.
Try It: The Speleothem Time-Lapse
Tap the button to advance time and watch how mineral-rich water slowly sculpts the cave over thousands of years.
Apply Your Knowledge
Let us see if you can apply these subterranean concepts.
Match the Concepts
Click an object to select it, then click the matching description to place it.
Real-World Challenge
Imagine you have discovered a brand new cave deep in the mountains. Inside, you find a small underground lake, but there are absolutely no connections to the surface, and no bats or outside animals can get in. Based on what you know about energy in a cave ecosystem, do you think you will find fish or insects living in this lake? Why or why not?
What Has Changed Since This Episode Aired
This episode first aired in 1998. While the core geology remains highly accurate, our understanding of cave science has expanded considerably. Here are three things we know now.
Updated: Scientists have since discovered deep cave ecosystems that do not rely on the sun or surface energy at all. In places like Lechuguilla Cave in New Mexico, unique chemoautotrophic bacteria survive by eating the rock itself, feeding off sulphur compounds to create acidic biofilms affectionately known as snottites. These bacteria form the base of an entirely subterranean food web.
Updated: Today, geologists do not just admire how old speleothems are; they read them like tree rings or ice cores. By analysing the oxygen isotopes and trace elements trapped in the mineral layers, scientists use stalagmites as highly accurate climate proxies to reconstruct ancient weather patterns, droughts, and temperatures from thousands of years ago.
Updated: Since 2006, North American cave-dwelling bats have faced a devastating fungal disease called White-nose syndrome. The cold-loving fungus thrives in the constant, cool temperatures of caves and awakens bats during winter hibernation, causing them to burn through their fat reserves and starve. This has fundamentally shifted cave conservation efforts to protect these vital trogloxenes.
Test Your Understanding
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Reflection
What surprised you most about the animals that live in complete darkness? If you were to explore a cave, what is the first thing you would want to look for?
Episode Discussion
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