Most caves form when slightly acidic water slowly dissolves soluble bedrock, carving out passages over tens of thousands to millions of years. Limestone is the most common host rock, and the dissolving agent is usually just rainwater that has absorbed carbon dioxide from the atmosphere and soil. But that classic process is only part of the story. Caves also form from sulfuric acid rising from deep underground, from flowing lava that crusts over and drains, and even from microbial activity eating away at rock surfaces.
The Classic Process: Rainwater Meets Limestone
The vast majority of the world’s caves exist in karst landscapes, regions where soluble rocks like limestone, dolomite, or marble sit beneath the surface. The process begins in the sky. Rainwater absorbs a small amount of carbon dioxide as it falls through the atmosphere. When it hits the ground and filters through soil rich in decaying plants, it picks up much more. The result is a weak carbonic acid solution. It is not strong enough to burn your skin, but given enough time, it is more than enough to eat through solid rock.
As this acidic water seeps downward through cracks, joints, and bedding planes in the limestone, it reacts with calcium carbonate, the mineral that makes up the rock. The calcium carbonate dissolves, and the cracks widen. Over thousands of years, trickles become streams, and hairline fractures become tunnels and chambers. The process is slow by any human measure, but geologically speaking, it is remarkably efficient.
How caves develop depends heavily on where the water sits relative to the landscape. Above the water table, in what geologists call the vadose zone, water flows downward under gravity through cracks and vertical shafts. It carves canyon-like passages and potholes. Below the water table, in the phreatic zone, water fills every available opening and moves laterally under pressure. Passages formed here tend to be rounder, tube-shaped, and often follow the path of least resistance along fracture networks. Research on fossil water tables in limestone shows that the phreatic zone produces rapid cementation and alteration of the rock compared to the vadose zone, which shapes both the pace and geometry of cave development.1Sedimentology. Phreatic Versus Vadose Meteoric Diagenesis of Limestones: Evidence From a Fossil Water Table
When Acids Rise From Below
Not all cave-forming water comes from the surface. In some settings, the dissolving agent rises from deep underground instead of trickling down from above. This is called hypogenic speleogenesis, and it produces some of the most spectacular caves on Earth.
One version involves mixing corrosion. Imagine two bodies of groundwater, both saturated with dissolved calcium carbonate and therefore not capable of dissolving any more rock on their own. When they meet and mix, the chemistry shifts in a way that makes the combined water aggressive again. Modeling of this process shows that in an unconfined limestone aquifer, meteoric water seeping down from above and deep-seated water rich in carbon dioxide pushing up from below create a mixing zone along their boundary, and that fringe of mixed water is where dissolution happens fastest.2Hydrology and Earth System Sciences. Early hypogenic carbonic acid speleogenesis in unconfined limestone aquifers by upwelling deep-seated waters with high CO2 concentration: a modelling approach The passages carved by this process often look different from those shaped by surface water. They tend to form isolated chambers and networks that are not obviously connected to surface drainage.
An even more dramatic version uses sulfuric acid instead of carbonic acid. When hydrogen sulfide gas rises from oil and gas deposits or volcanic sources deep below, it can dissolve in groundwater and oxidize into sulfuric acid. This acid attacks limestone aggressively, replacing the dissolved calcium carbonate with gypsum crusts and deposits. Researchers have documented this process in caves across the world, from the Central Andes of Peru, where gypsum deposits on cave floors and walls carry sulfur isotope signatures pointing to hydrogen sulfide oxidation, to Central Asia, where warm groundwater releasing hydrogen sulfide has carved upward-trending passages through the rock.3International Journal of Speleology. Manifestations of sulfuric acid speleogenesis in the Mulapampa travertine, Central Andes of Peru: evidence from the Gruta con Lago4Doklady Rossijskoj akademii nauk. Nauki o Zemle. The first occurrence of sulfuric acid speleogenesis in Uzbekistan (Central Asia) Carlsbad Caverns in New Mexico is one of the best-known examples of a sulfuric acid cave. The enormous rooms there were carved not by surface streams but by corrosive water working upward through the rock.
Caves That Form Without Dissolving Rock
Dissolution accounts for the majority of caves, but it is not the only game in town. Volcanic activity creates an entirely different kind of cave: the lava tube. When a river of basaltic lava flows downhill, the surface cools and solidifies while molten rock continues to move underneath. Eventually, the lava supply dwindles, the interior drains, and what remains is a hollow tube. Some lava tubes are short and cramped, while others stretch for kilometers.
Thermal and mechanical erosion play a significant role in how these tubes grow. In Hawaii, combined rates of thermal and mechanical erosion of roughly five to ten centimeters per day have been measured during active eruptions, and similar processes have been documented on Mount Etna.5ScienceDirect. Lava tube morphology on Etna and evidence for lava flow emplacement mechanisms Flowing lava essentially melts and scours the rock beneath it, deepening and widening the tube over the course of an eruption. Unlike limestone caves, which take geological ages to form, a lava tube can develop during a single eruptive event lasting weeks or months.
Caves in Salt and Gypsum
Limestone is not the only soluble rock. Salt (halite) and gypsum dissolve far more quickly in water, and caves in these materials form on dramatically shorter timescales. A salt cave can enlarge at rates measured in years rather than millennia, because halite is so soluble that even a modest flow of fresh water strips it away fast.
Laboratory and modeling work shows that because halite dissolves so rapidly, aggressive water entering a salt body from below tends to dissolve rock only near the point of inflow, creating conical or triangular-prism-shaped cavities rather than the branching passage networks typical of limestone caves.6Water Resources Research. Genesis and shape of natural solution cavities within salt deposits These cavities can expand quickly enough to cause land subsidence or collapse at the surface, making evaporite karst a real engineering concern in areas underlain by salt deposits. Gypsum caves sit somewhere between limestone and salt in terms of dissolution speed, but they share the tendency to develop rapidly and to pose ground-stability problems.
Microbes as Underground Sculptors
It might be surprising to think of bacteria and fungi as cave architects, but microorganisms play an active role in both dissolving and building cave features. Some microbes produce organic acids as metabolic by-products, and those acids can corrode limestone bedrock over time. Others drive chemical reactions that pull metals out of solution and deposit them as minerals on cave surfaces. The net effect is that microbial communities can shape caves in ways that go beyond what purely chemical processes would predict.7Elements. Cave Decorating with Microbes: Geomicrobiology of Caves
Experiments designed to mimic cave conditions have shown conspicuous calcite dissolution and precipitation linked to microbial activity. Under nutrient-poor conditions, microbes have even been observed becoming entombed in silica-rich precipitates, suggesting that biological involvement in silica mobilization at cave-like temperatures may be more widespread than previously thought.8International Journal of Speleology. Microbially-mediated carbonate dissolution and precipitation; towards a protocol for ex-situ, cave-analogue cultivation experiments This microbial influence is important for more than just cave formation. It also affects how secondary mineral deposits like speleothems preserve chemical signatures over time, which matters to anyone trying to use cave formations as records of past climate.
How Caves Decorate Themselves
Once a cave exists, the same chemistry that created it begins to fill it with formations. Stalactites, stalagmites, flowstone, and other speleothems all grow through the reverse of the dissolution process. Water that dissolved calcium carbonate on its way through the rock above eventually reaches the cave ceiling and drips into open air. The cave atmosphere contains far less carbon dioxide than the water does, so when that thin film of solution meets the cave air, dissolved carbon dioxide escapes rapidly, within seconds.9Chemical Geology. The impact of outgassing of CO2 and prior calcium precipitation to the isotope composition of calcite precipitated on stalagmites Losing that carbon dioxide shifts the water’s chemistry and makes it supersaturated with calcium carbonate. The mineral precipitates out, depositing a paper-thin layer of calcite. On the ceiling, drip by drip, a stalactite grows downward. On the floor beneath it, each fallen drop builds a stalagmite upward.
Stalagmites grow as isolated columns of calcium carbonate driven by the steady drip of supersaturated water from above.10PubMed Central. Shapes of ideal stalagmites Their shapes depend on drip rate, the diameter of the water splash on the floor, and the concentration of dissolved minerals in the water. A fast drip on a narrow spot builds a tall, slender stalagmite. A slow drip that spreads across a wide surface builds a broad, flat one. Over tens of thousands of years, some stalagmites grow several meters tall.
Reading the Walls and the Formations
Caves are more than geological curiosities. The shapes of their walls and the chemistry of their formations contain records of past environments that are hard to find anywhere else.
Cave walls often carry features called scallops: small, asymmetric hollows carved by turbulent water flow. Scallops are extremely valuable because their size is inversely related to the speed of the water that formed them. Short scallops mean fast water; long scallops mean slow water. Researchers use them to estimate past water velocities and discharges in passages where no one could safely observe the actual flooding.11International Journal of Speleology. Uncertainties associated with the use of erosional cave scallop lengths to calculate stream discharges By reading scallop patterns along a cave passage, you can reconstruct how flow changed over its length, which tells you something about the hydrology that shaped the cave in the first place.
Stalagmites, meanwhile, have become one of the most important archives for reconstructing ancient climate on land. The oxygen isotope ratios locked into each layer of calcite or aragonite reflect the temperature and rainfall conditions at the time that layer was deposited. Aragonite stalagmites are especially prized because they often allow exceptionally precise dating and fast growth rates that can approach near-annual resolution.12GSA Bulletin. Are aragonite stalagmites reliable paleoclimate proxies? Tests for oxygen isotope time-series replication and equilibrium By slicing a stalagmite in half, dating specific layers, and measuring the chemistry along the growth axis, scientists can build continuous climate records spanning hundreds of thousands of years, bridging gaps left by ice cores and ocean sediments.13Island Arc. Oxygen isotopes of the Japanese stalagmites as global and local paleoclimate proxies
Caves on Other Worlds
Earth is not the only place with caves. The Moon and Mars both show strong evidence of lava tubes, and these extraterrestrial caves may be far larger than anything found on our planet. Orbital imagery has revealed pit chains and collapsed skylights on both bodies that appear to be the roofs of subsurface tubes that partially caved in. A comprehensive dataset comparing terrestrial and planetary lava tubes suggests that those on the Moon and Mars are one to three orders of magnitude more voluminous than their Earth counterparts.14Earth-Science Reviews. Lava tubes on Earth, Moon and Mars: A review on their size and morphology revealed by comparative planetology That means some of these tubes could be wide enough to fit a small city inside.
The formation mechanisms are thought to parallel what happens on Earth, but with some key differences. On Earth, lava tubes form through overcrusting, shallow inflation of a lava flow, or deep inflation combined with thermal and mechanical erosion. On the Moon, where lower gravity and different eruption styles prevail, deep inflation and thermal erosion along weak rock layers appears to be the most plausible explanation for the truly gigantic tubes.15Space Science Reviews. Lava Tubes on Earth, the Moon, and Mars: Detection, Evolution, and Exploration Potential The lower gravity means wider spans of roof rock can support themselves without collapsing, allowing the tubes to grow to dimensions that would be structurally impossible here. These caves are attracting serious attention as potential shelters for future human habitation on the Moon and Mars, because they would offer natural protection from radiation, micrometeorites, and extreme temperature swings on the surface.
Why Some Landscapes Have Caves and Others Do Not
If dissolution is the main cave-forming process, the obvious question is why caves are not everywhere there is limestone. The answer comes down to a few interacting factors. First, the rock needs to be thick enough and chemically pure enough to support large voids without collapsing. A thin limestone layer sandwiched between shale will not develop the same passage networks as a massive, clean limestone formation hundreds of meters thick. Second, there need to be pre-existing weaknesses for water to exploit. Joints, faults, and bedding planes give water initial pathways. Without them, even very soluble rock dissolves slowly, because the contact area between water and rock stays small.
Third, and often underappreciated, is the availability of aggressive water over long periods. A region can have perfect limestone but produce few caves if there is not enough rainfall, if the water table is too deep, or if the climate has been too arid for too long. Conversely, tropical karst landscapes often host enormous cave systems because high rainfall and lush vegetation supply abundant carbon dioxide to the soil, keeping the water highly aggressive. The interplay between rock type, structure, climate, and time is what determines whether a landscape becomes riddled with caves or remains solid.
Human activity adds a modern wrinkle. Pumping groundwater lowers the water table and changes the zone where dissolution is most active. Conversely, injecting fluids underground, whether for waste disposal or energy extraction, can introduce aggressive water into formations that were previously stable. Urbanization over karst terrain carries real risks: sinkholes are essentially small-scale cave collapses, and they are more common in areas where water management has altered the natural flow. Understanding how caves form is not just an academic exercise for geologists; it has direct implications for anyone living, building, or drilling in karst country.