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In physical geology and karst geomorphology, a sinkhole—also known scientifically as a doline—is a closed depression, bowl-shaped hollow, or vertical shaft in the Earth's surface formed primarily by the chemical dissolution of underlying soluble bedrock. Sinkholes represent the diagnostic landform of karst topography, which develops extensively in terranes underlain by carbonate rocks such as limestone and dolomite, or evaporite deposits such as gypsum, anhydrite, and halite (rock salt). While some sinkholes form gradually over thousands of years as surface depressions, others collapse catastrophically within seconds, swallowing buildings, roadways, and vehicles without advance warning, making them substantial geological hazards.
The fundamental mechanism driving sinkhole development is chemical weathering through carbonation. As meteoric precipitation falls through the atmosphere and percolates through decaying organic matter in topsoil, it absorbs carbon dioxide to form weak carbonic acid (H2CO3). When this acidic groundwater encounters calcium carbonate (CaCO3) in limestone bedrock, a chemical reaction converts the insoluble mineral into soluble calcium bicarbonate [Ca(HCO3)2]. Over geological timescales, moving groundwater dissolves the rock along structural joints, fractures, and bedding planes, carving out expansive subterranean voids, conduits, and cavern systems. As these subsurface cavities enlarge, the overlying soil mantle gradually loses structural support.
Geologists classify sinkholes into three primary genetic categories: solution sinkholes, cover-subsidence sinkholes, and cover-collapse sinkholes. Solution sinkholes occur where bare limestone is exposed at the surface and dissolves gradually downward. Cover-subsidence sinkholes develop where permeable sandy overburden settles slowly into subsurface cavities. In contrast, cover-collapse sinkholes are sudden and catastrophic: cohesive clay soils form a structural roof over an expanding subterranean void until the weight of the soil arch exceeds its shear strength, triggering an immediate structural collapse. Famous sinkhole phenomena include the water-filled cenotes of Mexico's Yucatan Peninsula and marine blue holes such as the Great Blue Hole of Belize. In India, notable karst sinkhole and cave systems occur in the Borra Caves of Andhra Pradesh and the limestone caves of Meghalaya.