A sinkhole is a depression or hole in the ground caused by the collapse of the surface layer into a void beneath. These geological features are a natural part of landscapes where soluble rock, such as limestone, gypsum, or dolomite, lies near the surface. While some sinkholes are small, others represent immense natural events. The maximum size a sinkhole can achieve is dictated by complex geological forces that govern the stability of the Earth’s crust.
Geological Constraints on Sinkhole Size
The foundation for any large sinkhole is a specific type of terrain known as karst topography, which is characterized by the dissolution of soluble bedrock by slightly acidic groundwater. This slow, chemical weathering process creates a network of underground passages, caves, and voids. The size of the resulting sinkhole is ultimately limited by the structural integrity of the material covering these subterranean chambers.
A thicker, more cohesive layer of overlying sediment or rock allows a larger cavity to form beneath it before the surface collapses. If the rock above the void is strong, the chamber can grow to enormous proportions, supporting an immense ceiling for centuries. Conversely, if the overburden is thin or made of loose material, the surface will subside or collapse much sooner, resulting in smaller, shallower depressions.
Water flow, or the hydraulic gradient, also plays a significant part in determining the size limit. An aggressive flow of water helps to dissolve the bedrock more quickly and efficiently transport the dissolved material and any collapsed debris away from the void. This continuous cleaning action prevents the underground chamber from filling up, enabling it to expand to a much greater size.
Dimensions and Metrics of Sinkhole Scale
Quantifying a sinkhole’s size requires measuring three distinct dimensions: diameter, depth, and total volume. Most sinkholes are generally circular and vary in size from tens to hundreds of meters in both diameter and depth. Small to medium-sized sinkholes often range from a few feet to about 100 feet in diameter and a similar depth, providing a common baseline for comparison.
Diameter is the measured width across the surface opening, while depth is the maximum vertical distance from the rim to the lowest point. These two measurements do not always correlate directly; some sinkholes are wide and shallow, while others are narrow but incredibly deep. Volume, the most complete metric, is a calculation of the total space within the depression and is particularly useful for comparing the overall scale of irregularly shaped features.
The World’s Largest Sinkholes
The largest known sinkholes represent the maximum scale achievable under ideal geological conditions, reaching truly immense sizes. Among terrestrial examples, the Xiaozhai Tiankeng, or “Heavenly Pit,” in China is often cited as the deepest. This double-nested sinkhole measures approximately 2,054 feet long, 1,762 feet wide, and is an astonishing 2,172 feet deep, formed by the erosion of an underlying limestone cave system.
The scale of sinkholes is not limited to dry land, as submarine versions, known as blue holes, can also reach monumental size. The Taam Ja’ Blue Hole in Mexico’s Chetumal Bay is recognized as the deepest underwater sinkhole, with depths approaching 900 feet. The Great Blue Hole off the coast of Belize is circular, spanning more than a thousand feet across and over 400 feet deep. These marine features illustrate that the same dissolution processes occur beneath the sea, creating vast submerged caverns when the water level was lower during past ice ages.
Sudden Collapse vs. Gradual Expansion
Sinkholes achieve their final size through two fundamentally different growth dynamics related to the speed of their formation. The first is the cover-collapse sinkhole, which is the most dramatic and hazardous. This occurs when cohesive surface material, such as clay, forms a temporary arch over an enlarging subsurface cavity until the supporting structure fails. This results in a large, deep hole forming suddenly in minutes or hours.
The second and more common process is gradual expansion, seen in dissolution sinkholes and cover-subsidence types. These features grow slowly over decades or centuries as the underlying soluble rock dissolves or the surface soil settles into the growing void. The largest sinkholes, such as the massive tiankengs of China, represent the accumulated effect of this long-term, gradual dissolution process.