A long-forgotten nature pocket was uncovered inside a mainland Chinese mountain on May 6.

The sinkhole measured 306 meters (1004 feet) long, 150 meters (492 feet) wide, and 192 meters (630 feet) deep in Leye County, Guangxi Zhuang region. The cavity near Ping’e village astonishingly contained an ancient forest, home to several undiscovered tree and plant species.

An Institute of Karst Geology of the China Geological Survey team abseiled down more than 100 meters (328 feet) and took several hours to reach the pit’s bottom. They found three different entrances to the cave and measured some trees that soared nearly 40 meters (130 feet) with their branches extended towards the heavens above. Dense vegetation on the sinkhole floor grew up to shoulder height.

“I would not be surprised to know that there are species found in these caves that have never been reported or described by science until now,” Guangxi 702 expedition leader Chen Lixin said according to UNILAD.

World heritage site

The New Mexico-based National Cave and Karst Research Institute (NCKR) speculates geology, climate, and other factors slowly dissolved the mountain bedrock over many thousands of years.

Rainwater becomes slightly acidic from accumulating soil-borne carbon dioxide. When the liquid flows through cracks, it gradually erodes bedrock and creates tunnels or voids. Once caverns grow large, the ceiling collapses, becoming a sinkhole.

This process gave the interior an average volume exceeding 5 million cubic meters (176.5 cubic feet). This is deep enough to hold the Gateway Arch monument in St. Louis, Missouri.

“This is cool news,” NCKR executive director George Veni said, according to Live Science.

Southern China is well known for its so-called karst landscape filled with caves, sinkholes, fissures, and subterranean streams. Guangxi is particularly popular for sinkholes, rock pillars, natural bridges, and other formations. According to the United Nations Educational, Scientific, and Cultural Organization, the region has already been designated a world heritage site.

The region is home to at least 30 known sinkholes, and at the time of publication, the world’s largest sinkhole was in Fengjie County, Chongqing.

The already discovered Xiaozhai Tiankeng sinkhole (meaning a heavenly pit in Chinese) measures 2,000 feet long, 1,760 feet wide, and 2,100 feet deep. The natural formation boasts its own surreal waterfall.

“In China you have this incredibly visually spectacular karst with enormous sinkholes and giant cave entrances,” Veni said, according to UNILAD.

“In other parts of the world you walk out on the karst and you really do not notice anything. Sinkholes might be quite subdued, only a meter (3.2 feet) or two (6.5 feet) in diameter [and] cave entrances might be very small–so you have to squeeze your way into them,” he added.

More common than thought

The United States and other parts of the world also have sinkholes, except they are so small they are often overlooked. NCKR estimates a quarter of North America has karst or pseudokarst landscapes from either volcanic or wind activity. The institute also predicts cave terrain represents about 20% of Earth’s total landmass.

“Because of local differences in geology, climate and other factors, the way karst appears at the surface can be dramatically different so in other parts of the world you walk out on the karst and you really do not notice anything [and] sinkholes might be quite subdued–only a meter (3.2 feet) or two (6.5 feet) in diameter [plus] cave entrances might be very small so you have to squeeze your way into them,” Veni said according to Live Science.

Caves and sinkholes can support wildlife through deep underground water resources, commonly called aquifers. One West Texan cave became home to an abundance of tropical ferns after bats brought fern spores while migrating to Central and South America for the winter.

Karst aquifers already supply water to an estimated 700 million people worldwide. These resources are easily accessed and drained.

“Karst aquifers are the only types of aquifers that you can pollute with solid waste,” Veni said, according to the publication.

“I have pulled car batteries and car bodies, and barrels of ‘god knows what’–and bottles of ‘god knows what’ out of the active cave stream,” he added.

Hidden danger

The U.S. Geological Survey (USGS) revealed underground sinkholes could sometimes pose a serious health hazard because the void might not initially seem obvious to those standing above.

“Sinkholes rarely happen but, when they strike, tragedy can occur,” the USGS website said.

“It is a frightening thought to imagine the ground below your feet or house suddenly collapsing and forming a big hole in the ground,” it added.

Many of these collapses occur when soil can no longer support the land surface. This can happen if circulating groundwater naturally dissolves limestone, salt beds, carbonate rock, gypsum, anhydrite, dolomite, and other materials. These evaporite rocks cover about 35% to 40% of the United States. Spaces and caverns develop underground when these rocks dissolve.

“Sinkholes are dramatic because the land usually stays intact for a while until the underground spaces just get too big,” USGS said.

“If there is not enough support for the land above the spaces, then a sudden collapse of the land surface can occur. These collapses can be small, or … they can be huge and can occur where a house or road is on top,” it added.

The scientific agency confirmed sinkhole disaster-prone areas include Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania. These states are home to many regions where groundwater drains into the subsurface, creating subterranean voids that can be dangerous.

“Sinkholes can vary from a few feet to hundreds of acres and from less than 1 [foot deep] to more than 100 feet deep, [and] some are shaped like shallow bowls or saucers whereas others have vertical walls–some hold water and form natural ponds,” USGS said.

Natural terraforming

Limestone or dolomite reportedly dissolves most intensively in Florida, filling the Sunshine State landscape with “gently rolling” hills and shallow depressions.

Watertable movements can increase contact with rock surfaces and exposure to joints, fractures, bedding planes, and other weak points. When carbonate changes to liquid form, rainfall, and surface water can carry it away from the surface, leaving a small depression.

“On exposed carbonate surfaces, a depression may focus surface drainage, accelerating the dissolution process [and] debris carried into the developing sinkhole may plug the outflow, ponding water and creating wetlands,” the agency said.

These tend to occur where covering sediments contain sand and allow water to pass through. Areas where thicker cover material or sediments have more clay are less likely to develop sinkholes.

“Granular sediments spall into secondary openings in the underlying carbonate rocks.

A column of overlying sediments settles into the vacated spaces,” USGS said.

“Dissolution and infilling continue, forming a noticeable depression in the land surface.

The slow downward erosion eventually forms small surface depressions 1 inch to several feet in depth and diameter.”

Construction, development, groundwater pumping, and new water-diversion systems can also cause new sinkholes. Some sinkholes occur when the land surface is changed to create industrial and runoff-storage ponds, and the increased weight can trigger an underground collapse of supporting material.

“Overburden sediments that cover buried cavities in the aquifer systems are delicately balanced by groundwater fluid pressure [and] the water below ground is actually helping to keep the surface soil in place,” the agency said.

“Groundwater pumping for urban water supply and for irrigation can produce new sinkholes in sinkhole-prone areas [and,] if pumping results in a lowering of groundwater levels then underground structural failure–and thus sinkholes can occur,” it added.

Irregular shape

An earlier joint study by the University of Texas and the Institute of Karst Geology called “Emergency Investigation of Extremely Large Sinkholes” found most sinkholes have a round shape above the surface and a conical form underground in Maohe, Guangxi.

“Most sinkholes are circular or elliptical in shape on the surface and cone shape below [the] surface,” researchers said in their final report.

They found many sinkholes have an irregular shape on the Earth’s surface.

“The largest sinkhole is no. 11 with an irregular shape on the surface and a long axis of 70 meters (229.6 feet), short axis of 12 meters (39.3 feet) to 38 meters (124.6 feet), and a depth of 4 meters (13 feet),” they said.

“The smallest sinkhole is no. 30 with a diameter of 1.2 meters (3.9 feet) and a depth of 5 meters (16 feet),” they added.

Most sinkholes formed at the Shangmuzhao section of Maohe village have both northwest and southeast orientations. Water was turbid in a hand-dug well, and many air bubbles were released from the well. Total suspended solids were about 10% from pumped groundwater. The well collapsed four days later.

“Collapsing events were caused by soil piping and deformation due to highly fluctuating hydraulic conditions within the karst water system,” the report said.

“Karst features such as cave streams, large karst springs, karst windows, caves, and blue holes exist in the study area,” it added.

‘Water hammer’ effect

On-site investigations discovered collapses occurred during heavy rainfall. The rapid water level rose, causing the cave roof to collapse. This could trigger a “water hammer” effect in the karst conduit system.

“The ‘water hammer’ effect can release a pressure surge to the karst conduit system, causing severe soil damage and subsequent collapses [and] soil disturbance would change the hydraulic gradient, which can cause water-level fluctuations and eventually result in new sinkholes,” the report said.

Seepage deformations could also disturb red clay near the bedrock, potentially triggering more sinkhole collapses. Soil could take several years to stabilize to normal hydraulic conditions.

“Rapid water level rise after a storm event caused a series of collapsing events in Maohe village [and] several collapsing events were caused by extreme weather conditions,” the report said.

Researchers concluded large-scale sinkhole collapses in a dolostone karst area could cause severe damage. Despite conventional views, dolostone karstification is relatively weak compared to limestone settings.

They also found promising results by using ground-penetrating radar to monitor water pressure changes and turbidity.

“It is possible to predict future sinkhole collapses [and] these approaches are being used to prevent future sinkhole collapses, and to reduce the damage caused by the collapsing event,” the report said.

“Further studies of the relationship between an extreme weather event and sinkhole collapses need to be conducted to prevent such large-scale collapsing events in the future,” it added.

More regulation

A different joint study called “Methodological Considerations in Cover-Collapse Sinkhole Analyses” highlighted the need for greater regulation to address the dangers of sinkholes in Guangzhou City, Guangdong.

The Institute of Karst Geology and the Chinese Academy of Geological Sciences stressed the importance of better land-use planning to minimize death and damage from ground collapses.

“Groundwater dynamic monitoring data confirmed that the sinkholes in the study area were closely related to changes in groundwater levels [and,] therefore, the efforts which have been made to investigate and monitor the sinkhole development will be required to continue into the immediate future,” they said in their final report.

“Appropriate approaches are required prior to construction in order to understand the cover-collapse sinkhole genesis and its likely evolution,” they added.

Researchers compiled a detailed typology, morphometry, and chronology inventory of 49 cover-collapses. They used field surveys, micro-tremors, aerial photography, photogrammetry, ground-penetrating radar, electrical resistivity imaging, and natural source audio-frequency magnetotellurics. The effort confirmed a direct link between water and sinkholes.

“Cover-collapse sinkholes are the result of the downward water-borne transportation of soil or other related material into underlying voids in either limestone bedrock or other soil profiles,” they said.

“Cover-collapse sinkholes are characterized by roughly circular outlines, internal drainage, and distinct breaks in the land surface,” they added.

The study recommended that more frequent groundwater monitoring could help detect sinkhole events before they occur.

“A major proportion of recent cover-collapse sinkhole events have been induced by anthropogenic changes in hydrogeological systems,” it said.

“Monitoring of groundwater levels may become an effective method for capturing real-time changes in the underground hydrodynamic forces, and possibly even used to forecast the appearances of cover-collapse sinkholes,” it added.

Methodology

Field and photogrammetric surveys examined historical satellite remote sensing images, aerial photograph interpretations, and field surveys to analyze the morphometry and chronology of cover-collapses.

Microgravity, micro-tremors, seismic reflections, electrical resistivity tomography, electromagnetic 47 surveys, and other non-invasive geophysical techniques investigated subsurface cavities, porosity, fracture density, and water saturation that causes subsurface sinkholes changes.

Trenching, drilling, geophysical well logging, and other invasive techniques provided instant information on underground nature and geotechnical properties.

Hydrogeological monitoring and ground deformation monitoring helped identify what caused deformation and subsidence phenomena kinematics.

“These monitoring methods are necessary in cases of potential episodes of catastrophic collapse,” the report said.

Karstic terrain is arguably one of the most challenging natural geological hazards to test for development and construction. No known technique effectively resolves issues due to the high vertical and lateral variabilities of karst regions’ geological and hydrogeological characteristics.

“A small region with surface deformation issues located in south eastern China’s Guangzhou City was examined for the purpose of developing a methodological framework for the evaluations of the potential conditioning factors, which control the occurrences of sinkholes in soil-covered karst regions where karst evidence may be hidden,” the report said.

The study revealed the province recently recorded more than 400 large-scale karst cover-collapses, which affected more than 100 people. Many of these events resulted from dissolved Miocene carbonate at an elevation ranging between 30 meters (98.4 feet) and 80 meters (262.5 feet) above sea level.

“Evolution processes of the sinkholes appeared to be structurally controlled by the characteristics of the local and regional faulting [and] the most important tectonic feature in the area was the ‘Guangzhou-Conghua’ fracture–which was buried within the study area and recognizable only in part–with a typically 60 degrees northwest orientation,” it said. 

“Generally speaking, a large part of the investigated area was occupied by paddy fields, and buildings were relatively scarce [plus] the cover-collapse sinkholes were evidenced in the alluvial plane of the study area,” it added.

Multidisciplinary approach

Various multidisciplinary approaches were adopted to understand better what causes deformations, sudden catastrophic collapses, and subsidence phenomena kinematics.

They include: 

  • identifying surface subsidence and sinkhole features
  • monitoring groundwater dynamic conditions and deformations
  • detecting soil thicknesses and stratification and underlying subsidence features
  • precisely locating and defining subsurface karst features at depth, including cavities, conduits, and karst fissure zones.

Sinkhole information was collected and analyzed from local maps, public institutions reports, and other written documents during the initial phase. Rough details on alluvium thickness, ground elevations, and formation lithology were also gathered.

Detailed field surveys and color telephotographs were performed in selected sinkhole areas. Information was sought from locals and compared to weather conditions, well water level changes, and spatial and temporal sinkhole distributions.

Unmanned aerial vehicle platforms captured digital surface and terrain models for large-scale mapping. SenseFly mapping drones provided detailed and accurate surface elevation, and other geomorphological data, using Postflight Terra 3D software.

Historical Google Earth images helped gather recent morphological sinkhole changes, and

analyze Spatio-temporal distribution patterns between September 2014 and March 2015.

“Images from prior to 2014 showed no sinkholes in the area [and,] in addition, 47 collapse pits were identified in the aerial photographs from 2014 to 2015,” the report said.

“Two more sinkholes had formed in the area in October of 2016 and March of 2017, respectively.”

No casualties were recorded from these collapses. Only a rice harvest was partially lost, and some fruit trees were destroyed.

Ground-penetrating radar surveys helped locate and characterize information about soil cavities, active subsidence, and other sedimentological information. This helped identify color changes. A total of 20 surface-based profiles with a total length of 3 kilometers (1.8 miles) were conducted throughout the study. Cross-hole ground-penetrating radar effectively found many faults, caves, and other water-logged areas.

Micro-tremor exploration collected overburdened sedimentary layer thicknesses data, which showed the relationship between soil thickness and resonant frequencies in borehole drilling logs.

Electrical resistivity imaging differentiated different rock layers, while electrical resistivity tomography profiling identified shallow limestone deposits, large dissolution feature zones, and underlying cavities.

Audio frequency magnetotelluric methods detected karst fissure zone ranges through electrical conductivity in underground rock strata. 

Drilling provided valuable information on underground nature and geotechnical properties. It also detected soil caves, karst caves, karst conduits, sediments, and other voids.

Single-hole radar techniques recorded single-hole, full waveform radar data on the nature of reflectors distributed along boreholes.

Interferometric synthetic aperture radar technology scanned large areas for anomalous vertical movement and locations where significant changes typically occur. Mapping ground displacements also helped find potentially prone locations to future cover-collapse.

Field surveys with drones, aerial photogrammetry, and historical satellite remote

sensing images assisted in mapping sinkhole detailed inventory.

Drilling, micro-tremors, and electrical resistivity imaging identified soil layer thicknesses and structures. Drilling profiles showed quaternary soil layer thicknesses in the collapsed intensive area ranged between 9 meters (29.5 feet) and 14.2 meters (46.5 feet).

“To obtain a comprehensive understanding of the quaternary soil thicknesses in the study area, a contour map of the buried depths of the ground bedrock was obtained by utilizing a micro-motion inversion method,” the report said.

Most of the collapses occurred in areas where bedrock depths varied greatly.

“In the southwestern area of the site the bedrock was determined to be between 12 meters (39.3 feet) and 15 meters (49.2 feet) in depth [while] in the other areas of the site the thicknesses of the soil layers averaged approximately 10 meters (32.8 feet),” the study said.

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