In May 2022, a team of cave explorers discovered an extraordinary natural phenomenon in China’s Guangxi Zhuang Autonomous Region. They found a massive sinkhole, locally known as a “tiankeng,” measuring over 1,000 feet in length, nearly 500 feet in width, and 630 feet in depth. At the bottom of this giant cavity they found well-preserved ancient forest spanning nearly three football fields in length with trees reaching heights of up to 130 feet (40 meters)and dense undergrowth that stretches up to shoulder height.
“I wouldn’t be surprised to know that there are species found in these caves that have never been reported or described by science until now,” said Chen Lixin, the leader of the Guangxi 702 cave expedition team.
This giant sinkhole “heavenly pit,” in Chinese is located in south China’s Guangxi Zhuang Autonomous Region in the Leye County, bringing the number of giant sinkholes in the county to 30.
Southern China is well-known for its stunning karst landscapes, formed primarily by the steady dissolution of bedrock, such as limestone. According to NASA about 13% of China is covered by karst topography. Rainwater, which is slightly acidic, picks up carbon dioxide as it runs through the soil, becoming more acidic. Then, this acidic water flows through cracks in the bedrock, slowly turning them into underground tunnels and voids. Over time, when a cave chamber gets large enough, the ceiling can weaken and eventually collapse, opening up massive sinkholes.
This isn’t the first time Guangxi has amazed scientists. In November 2019, researchers discovered a cluster of giant sinkholes in the Guangxi region. Earlier, in 2016, scientists discovered the largest cluster of sinkholes in the world in Shaanxi Province, northwest China.
“Because of local differences in geology, climate and other factors, the way karst appears at the surface can be dramatically different,” said George Veni, the executive director of the National Cave and Karst Research Institute (NCKRI) in the U.S.
“In China you have this incredibly visually spectacular karst with enormous sinkholes and giant cave entrances and so forth,” Veni said. “In other parts of the world you walk out on the karst and you really don’t notice anything. Sinkholes might be quite subdued, only a meter or two in diameter. Cave entrances might be very small, so you have to squeeze your way into them.”
Types of Sinkholes
Sinkholes can form in different ways, and they are classified into three main types,
Solution Sinkholes:
Solution sinkholes typically form in areas where limestone or dolomite is exposed at the surface or covered by thin layers of soil and permeable sand. These sinkholes are created through the process of dissolution, where water gradually dissolves the rock. This process is most intense where water first contacts the rock surface or flows through preexisting openings such as joints, fractures, and bedding planes.
Dissolution is also accelerated in the zone of water-table fluctuation, where groundwater is in contact with the atmosphere. Over time, these processes can lead to the formation of small sinkholes. Solution sinkholes are generally slow to develop and relatively small in size compared to other types of sinkholes
Collapse Sinkholes:
Collapse sinkholes can form suddenly, sometimes within hours, and cause catastrophic damages. These sinkholes usually occur in areas where the covering sediments contain a significant amount of clay. The clay layer initially supports the surface, but over time, cavities form beneath it as the underlying rock, consequently, dissolves.
Eventually, the surface materials cannot support the weight, and the ground suddenly collapses. After an initial collapse, erosion and surface drainage often turn the sinkhole into a shallower, bowl-shaped depression. The sudden appearance and possibility of collapse make them one of the most dangerous types of sinkholes.
Subsidence Sinkholes:
Subsidence sinkholes form in areas where the ground above is made of permeable materials like sand. However in places where the cover materials are thicker or contain more clay, these types of sinkholes are less common and far more difficult to detect. They are usually smaller and may go unnoticed for a long period of time.
Sinkhole Locations in the world
Although sinkholes are found all across the world, they are most common in areas with large amounts of limestone, gypsum, or salt beds. The Dead Sea region, China’s karst regions, Mexico’s Yucatán Peninsula with its cenotes, and Florida and Kentucky in the United States are notable locations.
Karst areas are great for storing groundwater because the porous rock allows water to flow through it quickly, making it easy to collect and store. However, this also means they can get polluted easily since anything harmful, like chemicals or waste, can also travel quickly through the rock.
Karst aquifers are the primary source of water for almost 700 million people worldwide. The South China Karst is a Unesco World Heritage Site due to its distinctive karst landscapes and features.
