Everest stands tall, the Mariana Trench sinks deepest, yet the planet hides a giant hole that dwarfs them all—Xiaozhai Tiankeng, the deepest natural pit on Earth.
The extraordinary chasm lies in Fengjie County, Chongqing, China, with a diameter near 537 meters. It plunges between 511 and 662 meters, so vast that almost two Eiffel towers could fit inside. This colossal feature is known as Xiaozhai Tiankeng, or the Xiaozhai Heavenly Well, and it presents near-vertical walls and a total volume of about 119.3 million cubic meters. The formation is a twin structure, effectively two large bowls that rise to two distinct depths, each exceeding 300 meters.
At the base of the cliff, an underground river courses for roughly 8.5 kilometers beneath the surface. Before appearing above ground along the Migong River cliff, groundwater once created a 46-meter waterfall, proving the dynamic interplay between subterranean waters and exposed karst geometry.
Habitat for a wide variety of species
Beyond its sheer size, the sinkhole acts as a biodiversity sanctuary. It hosts 1,285 registered plant species, contributing to a rare ecosystem carved into the depths. Among its wildlife, the clouded leopard—one of the main inhabitants—is listed with a scant remaining population in the wild. Chinese flora such as Ginkgo biloba also features prominently in the surrounding karst landscape.
The Xiaozhai complex sits within a vast karst region composed mainly of Triassic limestones. Geologists estimate its formation occurred gradually over the last 128,000 years, marking it as relatively young when compared with other sinkholes globally, yet distinctly impressive in scale and beauty.
China hosts a remarkable cluster of large natural sinkholes, often called tiankeng, meaning heavenly well. Typical tiankengs reach at least 100 meters in depth and width, with rivers sometimes flowing beneath their rims. Rainfall and the dissolution of soluble rocks help sculpt these features, while underground rivers commonly emerge at their bases. The conditions required for tiankeng formation involve thick, clean carbonate or sandstone layers that persist above sea level in areas of heavy rainfall, allowing nature to carve these spectacular structures over time.
The result is a landscape that continues to evolve, drawing researchers and explorers who marvel at the scale and mystery of these natural pits.
China is not the only place where these wonders appear, but it hosts a significant share of the world’s largest tiankengs. Among the roughly 75 documented natural pits over 100 meters deep, about half lie in China, underscoring the country’s prominent role in this unique geological phenomenon.
2,800 steps to the bottom
In the Xiaozhai sinkhole, discovery remained quiet until scientists highlighted it in the late 20th century. The subsequent surge in tourism led to the creation of accessible routes into the interior. One notable route features a stairway with at least 2,800 zigzag steps that descend to the bottom. For many visitors, the climb back out—especially after a long descent—proves a decisive factor in choosing whether to explore to the rim or venture deeper.
As tourism developed, more pathways were opened, inviting curious travelers to witness the awe-inspiring scale, the unique microclimates, and the living tapestry of life supported within this vertical world.
Insights on the site emphasize responsible visitation and preservation, ensuring that future generations can experience the same wonder and ecological richness that define Xiaozhai Tiankeng.