curiosity about caves are infinitely magnified 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 the northwest of the former Yugoslavia, which is a limestone-covered area, and scholars from Yugoslavia were the first to study it. According to international convention, it was named after the place. In China, it is synonymous with karstification.
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 into 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 that formed at the same time as the surrounding host rock. Generally speaking, they are not related to groundwater. The main types are lava tubes (commonly known as volcanic caves) and travertine caves. Volcanic caves form due to the temperature difference between the inside and outside of flowing magma—the outside cools quickly, forming a hard shell, while the magma in the center remains hot and continues to flow, gradually forming passages.
Secondary caves refer to those where rocks formed first, and then various external forces acted upon the rocks to create the caves. 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 expeditions and research.
Semi-karst caves include, for example, the extensive conglomerate karst caves in Sichuan.
Caves formed by mechanical erosion include sea erosion caves created by waves crashing and eroding, aeolian caves in arid regions, and seepage erosion caves in soil layers, among others.
Based on the spatial morphology of caves (i.e., orientation), they are divided into four types: horizontal, inclined, vertical, and composite.
Caves are also classified by scale 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 scale. The achievements of a caver are also reflected through data on the length or depth of their cave exploration.
Formation of Caves:
Water and soluble limestone are the prerequisites for the formation of 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 support the weight of the ceiling, resulting in 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 formulas illustrate the principles of cave formation, but the process is reversible.
CaCO3 + H2O + CO2 → Ca(HCO3) (rock dissolves to form caves)
Ca(HCO3) → 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.
Stalactite Classification
In karst caves, the most direct resource is the stalactites. So, how do we distinguish among the dazzling array of shapes?
Gravity water deposits: These are the most common and numerous types of deposits in caves. They also have more detailed classifications...
Dripstone deposits include soda straws, stalactites, stalagmites, and columns. Soda straws grow downward, are very thin, hollow, and have clean, transparent white walls. Stalactites are the thicker, mature form of soda straws, typically cone-shaped. Stalagmites grow upward from the ground under the influence of dripping water, are solid inside, and often stack up like caps. Columns form when stalactites and stalagmites join together. Flowstone deposits come from water flowing along surfaces: those that descend from the cave ceiling are called "flowstones from above," such as draperies and cave curtains. Those that flow along cave walls are known as "wall flowstones," including rimstone dams and cave waterfalls. Those originating from the cave floor are called "floor flowstones," with rimstone dams being the most obvious example.
Pool deposits include rimstone, crystal flowers, cave pearls, moonmilk, and raft crystals. Rimstone often forms rimstone dams, which resemble flowstone dams but should be distinguished based on their formation process.
Non-gravity water deposits form from capillary water or films of water seeping from cave walls or stalactites. These include rare formations like helictites (twisted crystals) and cave hair. Their unusual appearance adds charm to the cave, which is why I like to call them “cave sprites.” They typically appear in more enclosed sections of caves.
It’s worth noting that some soda straws and stalactites may change direction during growth due to wind forces, so don’t be fooled by nature’s tricks.
Syngenetic deposits are formed through the interactive cooperation of two or more types of water flow with different movement patterns. Examples include shelfstones, palmate stalagmites, and crystal cones.
The formations not detailed above can usually be understood by their names. After distinguishing these various forms, let’s discuss why stalactites come in white, black, brownish-red, and other colors.
The color of stalactites is closely tied to their formation. Calcite can naturally appear white, yellow, or black, while gypsum can be white, yellow, or brown. Brownish-red hues are usually caused by the presence of ferric iron. If water flows through coal seams, it can result in completely black colors. Other colors may stem from oxidation-reduction reactions, weathering, human damage, and other factors. Casual cavers don’t need to know the exact causes, so we won’t delve deeper here.
When people discover stalactites, they often ask about their age. Stalactites can only form after a cave passage has emerged from a fully water-filled state, so they are generally much younger than the cave itself. Typically, the oldest stalactites in a cave are around 350,000 to 1,000,000 years old, often showing severe weathering and erosion. Those aged 150,000 to 300,000 years tend to be darker in color (brown or gray-black), with slight surface weathering but no flaking and have stopped growing. Stalactites 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 growth rates of about 0.6 to 1.2 meters per 10,000 years for stalagmites, which are the fastest-growing type of dripstone. Therefore, any form of damage is irreversible, irreparable, and unforgivable.
Regarding whether there are monsters or giant pythons in caves, let’s start with a simple principle:
As everyone knows, all life depends on sunlight. So, does the subterranean world without sunlight also teem with life like a pristine forest? The answer is definite: no. The deeper you go into a cave, the simpler and more primitive the food chain becomes. Therefore, it’s impossible for oversized creatures like aliens or giant pythons to exist (excluding human-made ones). Caves are full of massive rocks and soil—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 traits, cave animals can be divided into three categories:
True troglobites: These live exclusively in the dark and cannot survive outside caves. They often lack pigmentation, have degenerated or absent eyesight, and have slow metabolisms. Examples include blind cave 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. Examples include cave rats, earthworms, and certain salamanders.
Trogloxenes: These are temporary cave dwellers, such as bats, swifts, snakes, and some insects.
Recent reports and explorations have uncovered many more species. Who knows how many undiscovered creatures await us in the mysterious underground world?
These animals outline a basic food chain:
In water, cave tadpoles feed on microorganisms, blind cave fish eat the tadpoles, bats prey on the fish, and microorganisms decompose bat droppings and fish carcasses.
On land, cave crickets and centipedes feed on rat or bat droppings, while rats or bats may prey on the insects. Their remains and waste are then broken down by microbes.
Caves have their own climate phenomena, mainly regulated by flowing water and convective air. Caves are actually relatively constant-temperature environments. Elevation affects a cave’s average temperature, but it generally stays between 11–15°C. This is where the saying “cool in summer, warm in winter” comes from.
Air movement in caves, besides regulating the climate, also significantly impacts stalactite growth. Therefore, when exploring or developing caves, avoid casually breaking through passages, as the consequences can be devastating.
People entering caves often feel a sudden strong gust of wind in narrow passages, which then abruptly stops. This is due to the “chimney effect,” where temperature differences between the cave entrance (or cracks) and the outside cause natural air convection. The chimney effect shows strong seasonal variation: in winter, warm air rises upward from inside the cave, while in summer, the reverse occurs. In spring and autumn, when cave and outside pressures are similar, daily temperature changes cause fluctuating wind directions at different times of the day.
The flow of underground rivers also generates air movement, a phenomenon called the “riding effect.”
Speaking of which, we can now address a widely asked question: Are there toxic gases in caves? Is there enough oxygen?
The real concern is oxygen availability. Here’s some data: the average CO₂ level in the atmosphere is 0.03%. Environments with 3% or higher CO₂ levels are dangerous, causing symptoms like rapid breathing, increased pulse, headaches, dizziness, ringing in the ears, and flickering candle or match flames. Since CO₂ is 1.53 times denser than air, it tends to accumulate at the cave floor. However, not all cave floors have high CO₂ levels. Based on my experience, if a passage suddenly feels warmer or you detect a strange odor, it means the tunnel is blocked or poorly ventilated—so proceed with caution. Cool, well-ventilated, and water-flowing passages are generally safe.
Caves have high levels of negative ions, which stimulate the self-purification of the respiratory system and boost immunity. This is why people often feel energized upon entering a cave.
Caves also contain trace amounts of radioactive elements. Pure carbonate rocks have lower radioactivity than other types. However, those working or staying in caves for extended periods should still monitor cumulative exposure time. This is why developed caves undergo regular radiation monitoring.
The comprehensive physiological effects of the cave environment on the human body manifest in four main ways:
1. Disruption of biological rhythms—caves lack seasonal or temporal cues, disrupting the “biological clock” and elongating its cycle.
2. Climate chaos—without temperature or humidity variations, prolonged stays in caves may lead to a disoriented state.
3. Sensory chaos—extended exposure to a dark, silent environment deprives the senses of familiar stimuli, leading to...
Impaired function leads to nervous system tension, eventually resulting in hallucinations.
4. The effect of cold -- it triggers the body’s protective mechanisms for adapting to stress and extreme stress, producing both slow and rapid stress responses.
Human ancestors lived in caves for extended periods, so people likely have a special connection to them. The continuous discovery of cultural relics in caves demonstrates the long and close relationship between humans and these spaces.
The discovery of Peking Man in Zhoukoudian, Beijing, the cliff coffin tombs in the Three Gorges of the Yangtze River, cave paintings found across Europe, various artifacts unearthed from caves worldwide, as well as the widespread nitre pits in caves, all reflect humanity’s historical use of caves. Nitre pits are a unique phenomenon in Chinese caves, showing how ancient people extracted nitre from nitrous soil—highlighting their ingenuity while also reflecting the limitations of early industry.
In addition to the accumulation of cultural relics, caves also contain vast deposits of ancient biological remains, providing rich and detailed information for understanding history.
Protection of Caves
This is a critical issue. Currently, neither the national nor local governments have specific laws or regulations for cave protection, and conservation efforts remain largely at the grassroots level. Therefore, as cave explorers or enthusiasts, it is everyone’s responsibility to actively and voluntarily promote awareness and implement protective measures. In caves, apart from taking garbage and photos, nothing else should be removed. No vegetation should be damaged, no cave animals harmed, and no pollution introduced to cave water. When in groups, a single-file line is recommended. Abroad, even human waste is collected in special containers and carried out upon departure.
Cave development should follow principles of scientific planning and responsible exploitation. Practice has shown that hasty development without comprehensive surveys, mapping, and planning—coupled with insufficient protective measures—can lead to severe consequences for the resource. For developers, such practices also compromise the long-term commercial viability of cave resources.
Every cave is a treasure—not only for our enjoyment but also for future generations to appreciate. It is hoped that everyone who reads this will genuinely commit to cherishing and protecting caves.
How to Ensure Safety in Caves
When entering a cave, the first priority is to ensure sufficient lighting. Using reliable lamps and carrying ample batteries is a good practice. For longer expeditions, it’s essential to have at least two backup light sources. Wearing a helmet can help ensure safe movement underground (except for those who have trained—or claim to have trained—their heads to be iron-like).
When exploring unfamiliar caves, go in groups of at least two. If you get lost, avoid wandering aimlessly. Staying calm and waiting for rescue in place is the best approach.
Cave temperatures are low, so it’s important to stay warm during breaks. A sleeping bag with a fill weight of 400 grams or more is recommended. It’s also wise to bring a fleece garment.
Cave water must be boiled before drinking. If the water source is not very clean, filter it several times before boiling. Avoid wading through water when the currents are unclear and no protective gear is available. Take waterproofing precautions (especially for batteries and electrical equipment).
While moving in caves, do not run or jump. When ascending or descending slopes without the need for equipment, it’s better to use both hands and feet.
In unfamiliar passages, let the most experienced team member lead the way. Mark your route according to established guidelines along the way.
Each person should carry their own batteries, food, and other essentials.
Safety is the fundamental principle of any activity. A safety-conscious attitude not only protects yourself but also shows responsibility toward the team. Such individuals are always welcome.
Professional cave explorers should possess technical expertise, reliable gear, and strong teamwork skills.
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, at 1,710 meters deep
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, spanning 116,000 square meters
Human Response to Changes in Oxygen and CO2 Levels
Effect of Oxygen
20.9%: Optimal for breathing, normal flame combustion
19%: No discomfort felt yet, flame combustion reduced by one-third
17%: Causes increased heartbeat and breathing difficulties during strenuous work, though no effect when at rest; flame combustion ceases
15% oxygen deprivation in the human body, rapid breathing and pulse, impaired judgment, muscle function breakdown
12% noticeable oxygen deficiency
Effects of carbon dioxide
1% increase in breathing rate and depth
3% accelerated breathing and pulse, flickering flames
4% headache, dizziness, ringing in the ears
6% severe breathing difficulty, poor combustion in acetylene lamps
7–10% loss of consciousness within minutes, death, acetylene lamps extinguished