Caves have always been regarded as mysterious places in people's minds. Europeans believe caves are gateways to hell, while Tibetan people consider them evil places and rarely enter them. Movies depicting hidden treasures guarded by giant pythons, along with various mystical spells attached to them, have stirred both fascination and fear about caves. People often ask me: Have you encountered ghosts in there? Have you seen tigers? Have you found any treasure? And so on. These questions are not strange, because fear and curiosity about caves are greatly amplified in people's minds.
We should view caves dialectically. They have their frightening aspects, as exploring the underground world is truly an adventure, but they are also just normal geological phenomena, and many people have actually visited caves. I hope that through the following discussion, everyone can gain a preliminary and clear understanding of caves.
Speaking of caves, we have to start with karst landforms: Karst is the name of a plateau in northwestern former Yugoslavia, which is a limestone-covered area, and scholars from Yugoslavia were the first to study it. By international convention, it was named after the place. In China, it is synonymous with karst topography.
So, do karst caves represent all caves? Obviously not, they are just one type of cave. Let’s look at the definition of a cave according to the International Union of Speleology: a natural underground space that humans can enter. This distinguishes it from man-made mines, tunnels, and the like.
With research on caves, people have classified them quite comprehensively. Let’s give a brief introduction.
Based on the formation of underground spaces, they are divided into primary caves and secondary caves. Primary caves are underground spaces formed at the same time as the surrounding rock. Generally, they are not related to groundwater. The main types are lava tubes (commonly known as volcanic caves) and travertine caves. Volcanic caves are formed due to the temperature difference between the inside and outside of flowing magma—the outside cools quickly, forming a hard shell, while the central magma remains hot and continues to flow, gradually forming cave passages.
Secondary caves refer to caves formed after rocks have been subjected to various external forces. Let’s take a look at what these external forces are and what kinds of caves they form...
Caves formed by dissolution include karst caves and semi-karst caves. Karst caves are very common in China and are the main subjects of current speleological scientific research. Semi-karst caves include, for example, the extensive conglomerate karst caves in Sichuan. Caves formed by mechanical erosion include sea caves formed by wave erosion, aeolian caves in arid regions, and seepage caves in soil layers, among others. Based on the spatial morphology (i.e., direction) of caves, they are divided into horizontal, inclined, vertical, and composite types. Based on cave size, they are classified into four levels...
Small: length less than 50 meters, vertical depth less than 50 meters
Medium: length 50–500 meters, vertical depth 50–200 meters
Large: length 500–5000 meters, vertical depth 200–1000 meters
Giant: length greater than 50000 meters, vertical depth greater than 1000 meters
Based on water conditions, they are divided into dry caves and water caves, with water caves further divided into outflow caves and inflow caves.
Currently, the most popular classification method is by size. The achievements of a caver are also reflected by the data on the length or depth of the explored caves.
Cave Formation:
Water and soluble limestone are the prerequisites for forming karst caves. Water dissolves the soluble bedrock into calcium bicarbonate. Year after year, the cave passage space continues to enlarge until it can no longer bear the weight of the ceiling, leading to a collapse. The collapse only changes the shape of the space, not its size. The collapsed material is then carried away by the water flow, which continues to dissolve the bedrock. This cycle repeats, and eventually, a cave system is formed. The following chemical reaction formula explains the principle of cave formation, but it is reversible.
(Rock dissolves to form caves)
Ca(HCO₃)₂ → CaCO₃ ↓ + H₂O + CO₂ ↑ (Precipitation forms stalactites)
Karst caves also go through stages of formation, development, and decline. The formation of collapse sinkholes is a manifestation of cave development.
Classification of Stalactites:
In karst caves, the most direct resource is their stalactites. So, how do we distinguish among the dazzling array of shapes?
Gravity water deposits: These are the most common and numerous deposits in caves. They have even finer classifications...
Drip water deposits: Include soda straws, stalactites, stalagmites, and columns. Soda straws grow downward, are very thin, hollow, with white, transparent walls. Stalactites are the thicker, grown form of soda straws, usually conical. Stalagmites grow upward from the ground under the effect of dripping water, are solid inside, and form a stacked cap shape. Columns are formed when stalactites and stalagmites connect. Flowing water deposits—those flowing from the cave ceiling are called flowstone, such as draperies and curtains. Those flowing along the cave walls are called wall deposits, like rimstones and waterfalls. Those originating from the cave floor are called floor deposits, such as flowstone dams, which are most prominent.
Pool water deposits: Include rimstone, crystal flowers, cave pearls, moonmilk, and cave rafts. Rimstones often form rimstone dams, which resemble flowstone dams, though they should be distinguished based on different formation methods.
Non-gravity water deposits: Formed by capillary water or film water seeping from cave walls or stalactites. Examples include helictites (twisted stalactites) and cave hair—uncommon formations. Their appearance is quite interesting and adds charm to caves. I like to call them “cave sprites.” They generally appear in more enclosed sections of caves.
It’s worth noting that some soda straws and stalactites may change direction after growing for a period—this is caused by wind action, so don’t be fooled by nature’s tricks.
Any formations not specified in detail above can be understood by their names. After distinguishing the various types, we need to talk about why stalactites come in white, black, brownish-red, and other colors.
The color of stalactites is closely related to their formation. Among the possible colors of calcite are white, yellow, and black. Gypsum can also appear in white, yellow, and brown. The commonly seen brownish-red color is usually due to the presence of ferric iron. If water passes through coal seams, it can result in an entirely black color. Other colorations may be caused by redox reactions, weathering, human damage, and other factors. The average cave explorer doesn’t need to know the exact causes, so we won’t go into too much detail here.
When people discover stalactites, they often ask about their age. Stalactites can only form after a cave passage has become free from being fully water-filled, so they are generally much younger than the cave itself. Typically, the oldest stalactites in a cave are around 350,000 years old, but under 1 million years, and these often show severe weathering and erosion. Those aged between 150,000 and 300,000 years tend to have darker exteriors (brown or gray-black), with slight surface weathering but no flaking or layering, and growth has stopped. Those under 100,000 years old are lighter in color and have smooth, solid surfaces. The youngest generation is between 2,000 and 20,000 years old. In some caves, scientists have monitored that stalagmites typically grow at a rate of about 0.6 to 1.2 meters every 10,000 years, and among all types of stalactites, stalagmites deposit the fastest. Therefore, any kind of damage is irreversible, irreparable, and unforgivable.
Regarding whether there are monsters or giant pythons in caves, let’s start with a very simple principle:
As everyone knows, all living things rely on the sun. So, does the subterranean world without sunlight also teem with life like a primeval forest? The answer is definite: no. The deeper you go into a cave, the simpler and more primitive the food chain becomes. Therefore, it is definitely impossible for oversized creatures like aliens or giant pythons to exist (not counting those created by humans). The caves are full of huge rocks and dirt—what would such creatures eat if they lived there long-term? With this concept in mind, let’s talk about what kinds of organisms actually live in caves... Based on ecological characteristics, cave animals can be divided into the following three categories:
True cave-dwelling animals: These can only survive in the dark world of caves and cannot live outside in normal environments. They typically lack pigmentation, have degenerated or absent visual organs, and have slow metabolisms. Examples include cave blind fish, cave salamanders, cave crickets, cave centipedes, and cave tadpoles.
Troglophiles: These complete their life cycles in caves but can also survive in dark environments outside caves. Examples include cave rats, earthworms, and certain salamanders.
Trogloxenes: These are animals that temporarily inhabit caves, such as bats, swifts, snakes, and some insects.
In recent reports and explorations, many more animals have been discovered. In this mysterious underground world, who knows how many unknown creatures are waiting to be found and understood? The animals mentioned above outline a rough picture of the food chain:
In the water, cave tadpoles feed on microorganisms, cave blind fish eat the tadpoles, and bats prey on the blind fish, while microorganisms decompose bat droppings and the remains of blind fish.
On land, cave crickets and cave centipedes feed on the droppings of mice or bats, which in turn prey on the crickets and centipedes. The resulting corpses and droppings are decomposed by microorganisms.
Caves have their own climatic phenomena, mainly regulated by flowing water and convective air. The interior of a cave is actually a relatively constant-temperature environment. Altitude does affect the average temperature of a cave to some extent, but it generally ranges between 11-15°C. This is where the saying “cool in summer and warm in winter” in caves comes from.
The movement of air in caves, besides regulating the climate, also greatly affects the growth of stalactites. Therefore, when exploring or developing caves, do not easily break through passages—doing so often leads to devastating destruction.
People entering caves often feel a sudden strong gust of wind when passing through a narrow passage, but it instantly calms down once they’re through. This brings us to the “chimney effect,” which occurs when differences in height between the cave entrance (or fissure) cause natural convection of air with different temperatures inside and outside the cave. The chimney effect shows strong seasonal changes: in winter, the warm air inside the cave moves upward, while in summer, the opposite occurs. In spring and autumn, when the pressure difference between inside and outside the cave is minimal, daily temperature variations lead to changing wind directions at different times of the day.
The flow of underground rivers can also generate air movement, a phenomenon known as the “riding effect.”
Now we can answer a question that many people are concerned about: Is there toxic gas in caves? Is there enough oxygen?
What we should really be concerned about is whether there is enough oxygen. Let’s look at some data: the average CO₂ content in the atmosphere is 0.03%. When the CO₂ level reaches 3% or higher, it becomes dangerous for humans, with symptoms including rapid breathing, increased pulse, headache, dizziness, tinnitus, and flickering flames from candles or matches. Since CO₂ is 1.53 times denser than air, it often settles at the bottom of caves. However, not all cave bottoms have high CO₂ levels. Based on my experience, if a cave passage suddenly feels warmer or you notice an unusual smell, it indicates poor or blocked ventilation, and such areas should be approached with caution. Cool, ventilated, and water-flowing passages are generally safe.
Caves have high levels of negative ions, which can stimulate the self-purification function of the human respiratory system and boost immunity. This is also why people often feel excited upon entering a cave. Caves also contain trace amounts of radioactive elements, though pure carbonate rocks have lower radioactivity than other types. However, if you work or stay in caves for extended periods, you should still pay attention to the cumulative exposure time. This is also why developed caves require radiation monitoring.
The comprehensive physiological effects of the cave environment on the human body are manifested in the following four aspects:
1. Disruption of biological rhythms—without seasonal or temporal changes in caves, the “biological clock” is disrupted, and its cycle is prolonged.
2. Climatic chaos—without fluctuations in temperature or humidity, prolonged stays in caves can induce a state of confusion.
3. Sensory chaos—prolonged exposure to quiet, dark environments without the usual visual or auditory stimuli can impair functions, leading to central nervous tension and even hallucinations.
4. Cold effects—can trigger protective mechanisms in the human body for stress and hyper-stress, resulting in slow or rapid stress responses.
Human ancestors had long periods of cave dwelling, so humans likely have a special bond with caves. The continuous discovery of cultural relics in caves demonstrates the long and close relationship between humans and caves.
The discovery of Peking Man in Beijing’s Zhoukoudian, suspended coffins in cliff caves along the Yangtze River’s Three Gorges, cave paintings found across Europe, various cultural artifacts unearthed from caves worldwide, and the widespread nitrate pits in caves all reflect humanity’s historical use of caves. Nitrate pits are a unique phenomenon in Chinese caves, a method used by ancient people to extract nitrates from nitre soil, reflecting their ingenuity and the backwardness of industry at the time.
In addition to accumulated cultural relics, caves also contain large quantities of ancient biological remains, providing us with a wealth of detailed historical information.
Cave protection is a crucial issue. Currently, neither national nor local governments have specific laws or regulations to protect caves. Protective actions remain at the grassroots level. Therefore, as cave explorers or enthusiasts, we should actively and voluntarily promote awareness and implement protection efforts. In caves, apart from trash and photos, we should not take anything from inside. We must not damage any growth, hunt cave animals, or pollute cave water. In groups, single-file movement is advised. Abroad, special containers are used for human waste, which are taken out upon leaving.
Cave development should follow the principles of scientific planning and responsible development. Practice has shown that rushing into development without comprehensive surveys, mappings, and planning—and without protection measures—can lead to serious consequences for resources. For developers, cave resources will not have long-term commercial viability either.
Every cave resource is a treasure. Aside from our enjoyment, it is our duty to ensure future generations can also appreciate them. I hope everyone who reads this article will truly cherish and protect caves.
To ensure safe entry into caves, the first priority is having sufficient light. Using reliable lamps and bringing enough batteries is a good habit. For longer trips, it’s essential to prepare at least two backup lights. Wearing a helmet ensures your safety while moving through caves (unless you’ve trained—or are naturally gifted—with an iron head).
When entering an unfamiliar cave, there should be at least two people. If you get lost, do not wander around—stay calm and wait for rescue in place.
The temperature inside caves is relatively low. Make sure to stay warm when resting. A sleeping bag with a fill of 400 grams or more is recommended. It’s also a good idea to bring a fleece jacket.
Water in caves must be boiled before drinking. If the water source is not very clean, it’s best to filter it several times before boiling. Do not wade through water if you’re unsure of the conditions and lack protective gear. Take waterproofing measures (especially for batteries and electrical equipment).
When walking in a cave, do not run or jump. When there’s no need for equipment to go up or down slopes, try to use both hands and feet as much as possible.
In unfamiliar passages, let the more experienced team members go first. You can mark your route as required along the way.
Each person should carry their own batteries, food, and other items.
Safety is the basic principle of any activity. Having a safety awareness not only protects yourself but also shows responsibility to the team. Such individuals are welcome.
Professional cavers should possess technical expertise and reliable equipment, and demonstrate good teamwork.
World and domestic cave statistics
Longest cave system in the world: Mammoth Cave in the U.S., stretching 556,849 meters
Deepest cave in the world: Krubera Cave in Georgia, with a depth of 1,710 meters
Longest cave system in China: Shuanghe Cave in Guizhou, measuring 100,044 meters, currently ranked 16th in the world
Deepest cave in China: Qipeng Cave in Wulong, Chongqing, with a depth of 920 meters
Largest cave chamber in China: Miao Hall in Gebi River, Guizhou, covering 116,000 square meters