In this blog post, we’ll examine the principles behind sinkholes and ground subsidence—where the ground suddenly collapses—as well as their natural causes and those arising from urban development, and explore corresponding preventive measures.
A Bolt from the Blue on Dry Land: Sinkholes
Back in 2012, the ground collapsed like a bolt from the blue. Near a construction site for the Incheon Subway Line 2 in Incheon, the ground collapsed, creating a large hole in the road. It was a sinkhole. Sinkholes—something we had only seen in books, movies, or international news—had finally appeared in South Korea. At the time, in Wanggil-dong, Seo-gu, Incheon, the road surrounding the Incheon Subway Line 2 construction site collapsed extensively, resulting in the death of a motorcycle rider. Since then, the unfamiliar phenomenon of sinkholes has come to be perceived as a major source of fear among the public. What exactly is the nature of this hole, which resembles a monster capable of swallowing everything on the ground in an instant?
A sinkhole generally refers to a depression in the terrain caused by the ground sinking or suddenly collapsing. Sinkholes appear in various sizes and shapes around the world, and some naturally formed ones can reach enormous sizes, spanning hundreds of meters. For example, the Xiaozhai Tiankeng in Chongqing, China, is a world-famous giant sinkhole measuring approximately 626 meters in length, 537 meters in width, and 662 meters in depth. Giant sinkholes found in natural landscapes are often regarded as awe-inspiring geological phenomena. However, ground subsidence occurring in urban areas can be a disaster that brings fear and damage rather than wonder. To properly understand the nature of sinkholes and ground subsidence, it is essential to clearly identify their causes.
What happens underground?
Sinkholes generally occur when an underground void expands, causing the ground supporting the surface to collapse. In particular, in areas where water-soluble rocks such as limestone or dolomite are widely distributed, groundwater can dissolve the rock over a long period of time, creating cavities and caves underground. When these cavities grow large enough and the overlying soil can no longer bear the weight, the ground surface can suddenly collapse. Naturally occurring sinkholes are common in such karst landscapes. In contrast, in regions like South Korea, where granite and gneiss are widely distributed, the typical naturally occurring sinkholes found in limestone areas are relatively rare. However, this does not mean that ground subsidence does not occur in South Korea. In urban areas, ground subsidence and underground voids can occur due to various causes distinct from natural sinkholes.
One might question whether groundwater loss is significant enough to cause massive ground subsidence. However, the role of groundwater should not be underestimated. When water is present underground, it creates pore water pressure in the surrounding soil and strata; changes in the groundwater level also cause changes in the stress on the ground and in the flow of water. In a static water condition, water pressure increases by approximately 1 atmosphere for every 10 meters of depth; therefore, a pressure difference of about 2.5 atmospheres occurs at a depth of 25 meters, and about 25 atmospheres at a depth of 250 meters. Consequently, if the groundwater level drops sharply or the flow of groundwater changes, the stability of the ground may be compromised, potentially causing existing underground cavities to collapse or the ground surface to subside. In fact, it is well known that when the groundwater level drops due to excessive abstraction, the conditions supporting underground cavities change, which can lead to sinkholes or ground subsidence.
Under natural conditions, groundwater flow and the groundwater network can change due to tectonic movements or climate change. In limestone regions, calcium carbonate—the main component of limestone—dissolves in groundwater, creating voids underground and potentially causing the ground to sink. These processes often unfold over long periods of time. However, in urban areas, human activities can rapidly alter the subsurface environment. During large-scale development projects, drawing groundwater or conducting excavation work can alter groundwater flow and water levels, and the construction of tunnels or underground structures can affect the stability of the surrounding ground. Additionally, if aging water and sewer pipes rupture and leak, surrounding soil may be washed away or cavities may form underground, weakening the ground. As such, ground subsidence in urban areas can result from a complex interplay of various factors—including changes in groundwater, as well as excavation, utility installation, pumping, and the aging of underground infrastructure.
Ground Subsidence in South Korea and Preventive Measures
Recognizing that sinkholes do not occur solely due to natural causes—but that ground subsidence can also result from human activity—makes it clear that South Korea is also a region requiring ongoing safety management regarding ground subsidence. In fact, while not all ground subsidence occurring in urban areas can be classified as “sinkholes” in the natural sense, underground cavities and ground collapses can cause direct damage to roads, buildings, and underground facilities. The Seoul Metropolitan Government has also recently begun separately tracking the status of ground subsidence incidents, and from 2022 to 2024, a total of 59 cases of actual ground subsidence were recorded in Seoul. Since the number of reports may include complaints unrelated to ground subsidence—such as potholes or road damage—one should not conclude that actual ground subsidence has increased based solely on the number of reports.
Given that a significant portion of urban ground subsidence in South Korea is linked to human activity, fundamental solutions involve curbing indiscriminate urban development and systematically managing underground safety. When developing urban areas, the ground conditions and groundwater flow in the area must be thoroughly investigated, and changes in groundwater levels and the condition of the surrounding ground must be continuously monitored throughout the construction process. Another important preventive measure is to inspect and maintain existing water and sewer pipes that are aging or damaged and could potentially affect the surrounding ground. In particular, large-scale excavation projects and underground space development in urban areas can alter the surrounding ground and groundwater environment, so they require even more careful management. Currently, in South Korea, institutional management is in place under the “Special Act on Underground Safety Management” to prevent ground subsidence that may occur during underground development and the use and management of underground facilities, and to ensure underground safety.
Sinkholes and ground subsidence are not simply phenomena where large holes suddenly appear in the ground. Underground, various processes—such as groundwater flow and changes in water levels, rock dissolution, soil movement, the formation of underground cavities, and excavation and installation of underground facilities—interact and influence one another. These changes, which are usually invisible, can suddenly undermine the stability of the ground, causing the roads we walk on and the spaces where we live to suddenly sink. Therefore, to prevent ground subsidence, it is not enough to simply repair the damage after an incident occurs; it is crucial to investigate the condition of underground spaces in advance and continuously monitor changes over time. Strengthening the management system for the safe development and use of underground spaces is ultimately the most important way to protect people and cities from sudden ground collapse.
