A sinkhole is a depression or hole in the ground that forms when the surface layer collapses into a void beneath it. These geological features occur naturally across the globe, especially where the underlying rock is soluble. Determining the “biggest” sinkhole is complex because they exist both on land as massive pits (tiankengs) and underwater as deep marine caverns (blue holes). Since different features hold records for distinct measurements—such as depth, width, or total volume—accurate identification requires separate consideration for marine and terrestrial examples.
The Geology of Sinkhole Formation
Sinkholes develop primarily through karstification, a natural process involving the chemical dissolution of specific types of bedrock. This bedrock is typically composed of soluble rocks, such as limestone, dolomite, gypsum, or salt beds. The process begins when rainwater absorbs carbon dioxide from the atmosphere and soil, forming a weak carbonic acid.
This slightly acidic water percolates downward through cracks in the soluble rock. Over thousands of years, the water slowly dissolves the rock matrix, enlarging fissures into an underground network of conduits and caverns. As these subterranean voids expand, they eventually become too large to support the overlying soil or sediment. The surface material then collapses suddenly or subsides gradually into the empty space below, creating the characteristic depression of a sinkhole.
Measuring Size: Depth, Width, and Volume
Geologists use three primary metrics to quantify the size of a sinkhole. The first metric is absolute depth, which measures the vertical distance from the rim of the opening to the bottom of the pit or cavern. The second metric is the width, or diameter, which is the horizontal distance across the opening at the surface level.
The third metric is total volume, which represents the total amount of material dissolved and removed to create the void. Different sinkholes achieve dominance in separate categories; for example, one may be the deepest while another has a larger volume despite being shallower. This specialized classification allows for accurate comparison between various geological formations.
The World’s Deepest Marine Sinkhole
The deepest known marine sinkhole, or blue hole, is the Sansha Yongle Blue Hole, located in the Paracel Islands in the South China Sea. Also called the “Dragon Hole,” this submerged vertical cave descends to a measured depth of 300.89 meters (987.2 feet). Its surface opening is approximately 130 meters (426 feet) in diameter, giving it a nearly cylindrical shape.
Exploration revealed a distinct stratification of the water column and unique environmental conditions. Researchers found that water below about 100 meters (330 feet) is anoxic, meaning it lacks dissolved oxygen. This absence of oxygen inhibits most marine life, confining over 20 species of fish and other organisms to the oxygenated upper section. The blue hole formed during past ice ages when lower sea levels exposed the limestone, allowing rainwater to dissolve it, similar to land-based sinkholes.
The Largest Terrestrial Sinkhole
The largest terrestrial sinkhole in the world by total volume is the Xiaozhai Tiankeng, or “Heavenly Pit,” located in Fengjie County, China. This massive feature measures 626 meters (2,054 feet) in length and 537 meters (1,762 feet) in width at its rim. Its total depth ranges from 511 to 662 meters (1,677 to 2,172 feet) and features sheer, near-vertical walls.
Its total volume is estimated to be 119 million cubic meters of material, removed by an underground river system. The sinkhole is a double-nested structure, with the upper bowl descending about 320 meters and the lower bowl dropping an additional 342 meters. This vast pit hosts a unique, isolated ecosystem containing a primitive forest, a network of caves, and over 1,285 species of plants, including rare fauna like the clouded leopard.