Karst is a type of landscape shaped by the slow dissolving of soluble bedrock, most commonly limestone. Over thousands to millions of years, slightly acidic rainwater eats away at the rock, creating a distinctive terrain of sinkholes, caves, underground rivers, and springs. About 15% of the Earth’s ice-free land surface sits on the carbonate rocks that produce karst, and roughly a quarter of the world’s population depends on karst aquifers for drinking water.
How Karst Forms
The process starts with rain. As water falls through the atmosphere and filters through soil, it absorbs carbon dioxide and becomes mildly acidic. When this water meets soluble rock, it dissolves the stone grain by grain, widening natural cracks and fractures over time. What begins as hairline fissures gradually becomes a network of channels, then tunnels, then full cave systems.
Limestone is by far the most common rock involved, but karst can also form on marble, gypsum, and even quartzite. The speed of the process depends on how soluble the rock is, how much rainfall the area receives, and how much carbon dioxide is in the soil. Tropical regions with heavy rainfall and thick vegetation tend to produce the most dramatic karst landscapes, while arid regions develop karst features much more slowly.
Surface and Underground Features
Karst landscapes have a recognizable set of features, both above and below ground. On the surface, the most common sign is sinkholes: bowl-shaped depressions where the ground has collapsed into a cavity below or where rock has dissolved away from the top down. Some sinkholes are a few feet across, others span hundreds of meters. Disappearing streams are another hallmark. A river flows along the surface, then vanishes into a hole in the bedrock, continuing its path underground before resurfacing at a spring miles away.
Underground, the dissolved rock creates caves ranging from tight crawlways to enormous chambers. Inside these caves, mineral-rich water dripping from ceilings builds stalactites (hanging down) and stalagmites (growing up from the floor) over tens of thousands of years. Springs, where underground water returns to the surface, are often unusually large in karst regions because the water has been funneled through wide conduits rather than seeping slowly through porous rock.
Why Karst Matters for Water
Karst aquifers behave very differently from typical groundwater systems. In most rock, water filters slowly through tiny pores, which naturally removes many contaminants. In karst, water rushes through wide fissures and cave-like conduits with almost no filtration at all. Three factors make karst water supplies especially vulnerable to contamination: the high permeability of the dissolved rock, the speed at which water moves through the system, and the near-total absence of natural purification along the way.
Anything spilled on the surface of a karst landscape, whether fertilizer, sewage, or industrial chemicals, can reach the water table quickly, sometimes within hours. Even where no obvious sinkholes or streams are visible, contaminants seep into the porous upper layers of the karst and travel downward through cracks in the bedrock. This makes land use decisions in karst regions particularly consequential. A contamination event that might be contained in other geological settings can spread rapidly and unpredictably through a karst aquifer.
Life in the Dark
Karst caves are home to some of the most specialized creatures on Earth. The permanent darkness underground eliminates plant growth, creating an environment with very little food, stable temperatures, and near-total humidity. Species that have adapted to these conditions over millions of years, called troglobites, spend their entire lives underground and often cannot survive on the surface at all.
These animals have evolved in striking ways. Many have lost their eyes entirely, along with any skin pigment, since neither serves a purpose in total darkness. To compensate, they’ve developed elongated limbs and enhanced sensory structures to navigate and find scarce food by touch and chemical detection. Their metabolisms run slowly, and they reproduce at low rates, an adaptation to environments where calories are hard to come by. Many troglobites have also lost the ability to prevent water loss through their skin, which means they can’t survive in drier habitats outside their caves.
Because cave systems are isolated from one another, many species exist in only a single cave or a small cluster of connected caves. Central Texas alone is home to more than a dozen endangered karst invertebrates, including cave spiders, harvestmen, pseudoscorpions, and beetles found nowhere else on Earth. In Hawaii, the Kauai Cave wolf spider occupies tiny rock spaces adjacent to larger cave passages. This extreme geographic restriction makes karst species highly vulnerable to habitat disturbance.
Building on Karst
Construction on karst terrain comes with a set of engineering problems that don’t exist in most other landscapes. The bedrock surface beneath the soil is rarely flat. Instead, it forms an uneven pattern of peaks and grooves where limestone has dissolved unevenly. When a building’s foundation spans both solid rock pinnacles and soil-filled gaps between them, different parts of the structure settle at different rates. This differential settling produces cracks in walls, foundations, and floors, and in serious cases can compromise the structural integrity of the entire building.
A more dramatic risk is subsurface erosion, called piping, where water flowing through the karst gradually washes soil downward through cracks in the bedrock. This creates hidden cavities beneath roads and buildings. Because pavement and foundations act as a temporary roof over these growing voids, the problem may not be visible until a large cavity has formed. The result can be gradual subsidence or sudden collapse. Leaking water mains and storm drains accelerate the process by channeling extra water into the karst system, and lowering the water table through excessive pumping makes things worse.
Collapse sinkholes are the most sudden and dangerous hazard. They form when the thin layer of soil and rock above an underground cavity can no longer support its own weight and drops into the void below. These collapses can swallow cars, sections of road, and parts of buildings with little warning. In karst-prone areas, geotechnical surveys before construction typically include drilling to map the bedrock surface and identify potential voids, though hidden cavities are not always detectable.
Where Karst Is Found
Karst landscapes exist on every continent. Some of the most visually spectacular examples include the tower karst of southern China, where limestone has eroded into dramatic freestanding pillars and cones, and the Dinaric karst along the Adriatic coast of southeastern Europe, which gave the phenomenon its name (from the German spelling of the Kras Plateau in Slovenia). Florida sits almost entirely on karst, which is why sinkholes are a recurring problem across the state. The Yucatán Peninsula in Mexico is underlain by karst, producing the cenotes, natural swimming holes formed by collapsed cave roofs, that the ancient Maya used as water sources. Kentucky’s Mammoth Cave, the longest known cave system in the world at over 400 miles of surveyed passages, formed through karst processes in thick limestone bedrock.
In less dramatic forms, karst influences landscapes across large portions of the central and eastern United States, the British Isles, the Mediterranean, Southeast Asia, and southern Australia. Many people live on karst terrain without realizing it, since not all karst produces obvious surface features. In some areas, a thick layer of soil conceals the dissolved bedrock below, and the only clues are springs, subtle depressions in the ground, or the behavior of local streams that seem to lose water as they flow downstream.