In early 2022, engineers conducting land surveys in Leye County, in the Guangxi region of southern China, identified a large depression in the terrain through satellite scanning. The feature was significantly larger and deeper than any documented geological formation in the area. Local residents had long referred to the site as “heaven’s mouth,” and stories told of birds disappearing when they flew over it and cool air currents rising from the ground. The reports had been treated as folklore, but the survey data indicated the legends had a factual basis.
When the engineering team reached the site, they found a sinkhole measuring approximately 306 meters in length, 150 meters in width, and nearly 200 meters deep. The opening was large enough to contain a 60-story building. The floor was entirely dark even during the day. Initial attempts to survey the interior with camera-equipped drones failed because the devices lost signal transmission at depth and their cameras could not capture images in the darkness.
To gather information about potential hazards and the stability of the surrounding area, the team began pumping water out of the cavity while using scanners to map the underground walls. Specialists from across China, including researchers from the China Geological Survey, joined the operation. As the water receded, the crew detected a faint green color at the bottom, which raised the possibility of plant life in the depths.
The Guangxi region is known for its karst landscape, which forms when rainwater absorbs carbon dioxide and becomes mildly acidic. Over long periods, this water dissolves limestone and other soluble rock, creating caves and underground channels. When the roofs of these cavities become too thin to support their weight, they collapse, forming steep-sided depressions known locally as “tiankeng,” or heavenly pits. Guangxi has the highest concentration of these formations in the world.
In May 2022, a team of engineers, cavers, and biologists descended into the sinkhole in Leye County. The temperature dropped noticeably as they went deeper, and daylight disappeared. Using headlamps, they saw thick fog and three large cave entrances in the walls. After several hours of descent, they reached the floor at about 192 meters below the surface.
They found thick, moist soil rather than solid rock. The ground conditions suggested biological activity. Oxygen levels at the bottom were higher than expected for an enclosed space, indicating active photosynthesis by plants. The team’s lights revealed a fully developed forest. Trees reached heights of up to 40 meters, and the floor was covered with ferns, moss, and long vines. The environment had no visible signs of human impact.
The dense canopy created a self-sustaining microclimate with stable temperature and humidity. Mineral-rich water dripping from the walls and sediment falling from above had built fertile soil over time. Sunlight reached the forest floor through the opening, providing enough light for growth without direct exposure. Small streams crossed the landscape, and groundwater circulation beneath the floor appeared to support the vegetation.
The interior walls of the sinkhole were smooth rather than jagged, which indicated slow formation through water erosion rather than a sudden collapse. Seismic scanning revealed tunnels and chambers extending beyond the visible walls, suggesting a broader underground network possibly linked to rivers and caves. Water samples showed chemically stable groundwater with continuous movement, indicating an active underground river beneath the forest floor. No new cracks or signs of shifting were detected in the limestone walls, suggesting the structure had reached a stable state.
Jiang Yu Hai, a senior engineer from the Institute of Karst Geology of the China Geological Survey, joined the investigation. He identified three cave entrances at different elevations, all with steady airflow indicating an active system. The largest entrance, near the bottom of the sinkhole, contained mineral deposits along its walls.
Two weeks later, Jiang returned to find unauthorized workers installing platforms for tourist access. Drilling had caused cracks in the limestone walls spreading outward from the drill holes. Jiang ordered the work to stop, but expected that it would resume. He filed a report warning that continued drilling could destabilize the structure, leading to the collapse of the underground forest and the surrounding formation.
