A landscape dissolving from below
The Guangxi Zhuang Autonomous Region in southern China is famous for its almost surreal landscape of limestone towers. This scenery is part of the South China Karst, a UNESCO World Heritage site is the world's best example of humid tropical to subtropical karst landscapes. This terrain, formed from soluble rocks like limestone and dolomite, covers an area of about 550,000 square kilometers across the region. The landscape is the product of millions of years of geology; the limestone itself was deposited in a former ocean 250 to 400 million years ago.
The same process that creates the dramatic peaks, known as fenglin (tower karst) and fengcong (cone karst), also creates a hidden and hazardous subterranean world. Rainwater, made slightly acidic by absorbing carbon dioxide from the atmosphere, seeps into the ground and dissolves the bedrock. This process carves out a complex network of underground rivers, caves, and voids. When cities and infrastructure expand over this honeycombed foundation, the risk of sudden ground collapse, or sinkholes, becomes a serious concern.
Seeing the invisible threat
Urban development can accelerate the formation of sinkholes. The weight of buildings adds stress, while pumping groundwater for industrial or residential use can lower the water table. This removes the buoyant support that water provides to the roofs of underground cavities, making a collapse more likely. In one 2012 incident in Nanning, the regional capital, the ground sank after a well was drilled, forcing the evacuation of 844 villagers and causing one building to collapse.
To map this subsurface threat, geologists employ geophysical methods like Ground-Penetrating Radar (GPR). GPR sends electromagnetic waves into the ground and analyzes the reflections to create an image of what lies beneath, revealing hidden voids without any drilling. A survey along a 12-kilometer section of the Guilin-Yangshuo highway showed the scale of the issue. The GPR scan detected 337 subsurface voids. These cavities ranged from 1 to 6 meters in diameter and were found at depths of 1 to 5.5 meters, with a density as high as 48 voids per 100 meters of road. This detailed mapping allows for targeted intervention, such as grouting isolated voids, to prevent catastrophic collapses.