China is one of the world’s most biodiverse countries. Its vast territory contains an unparalleled range of ecosystems — from tropical jungle and coral reefs in the south, to icy, high-altitude deserts in the north.

But the country is also home to a unique, little-known habitat that is just starting to reveal its secrets: the tiankeng, or “heavenly pit.”

Tiankeng are enormous sinkholes — hundreds of meters deep — hidden among the jagged karst mountains that cover much of western China. Many of these sites have lain untouched for hundreds of thousands, sometimes millions of years, allowing unique ecosystems to evolve inside them.

Until recently, most of the tiankeng were unknown to science. Despite their enormous size, they’re often hard to spot amid broken and overgrown mountainous terrain. But dozens of them have been discovered over the past few years by Chinese cavers, some of whom hunt for new sinkholes using Google Earth.

These sites have generated huge excitement among Chinese scientists. Many of the tiankeng have become refuges for a large number of endangered plants and animals.

Researchers have even found a series of species inside tiankeng that previously were assumed to be locally extinct. New species — and sinkholes — are still being discovered on a regular basis.

Some scientists have started referring to tiankeng as China’s “biodiversity library.” They have hopes that these giant pits could help with efforts to restore native ecosystems, or shelter species from the effects of climate change.

These unique ecosystems, however, are also in grave danger. To local communities and authorities, there is often a huge temptation to turn tiankeng into tourist attractions. Several sinkholes have already been converted into sightseeing spots.

Unless stronger protections for tiankeng are introduced, it’s possible that a treasure trove of biodiversity will be lost forever.

Why China has so many ‘heavenly pits’

Like all sinkholes, tiankeng are pits formed when the surrounding rock collapses inwards. But according to Zhu Xuewen, a researcher at the Chinese Academy of Geological Sciences’ Karst Geology Institute, there are several features that make tiankeng unique, enough to separate them into their own geological category.

First, they are huge. According to Zhu, to qualify as a “heavenly pit,” a sinkhole must be at least 100 meters wide and deep. It also must have steep sides and have — or have once had — an underground river flowing along the bottom.

The river is what gives rise to a tiankeng. Flowing underground through limestone, the river slowly dissolves the surrounding rock, creating a cave which expands toward the ground surface and eventually breaks through it. The ground then collapses inward, forming a giant sinkhole — a tiankeng.

Limestone is required for tiankeng to form, and China has more limestone than any other country in the world. Almost a quarter of China’s territory — around 2 million square kilometers — consists of karst, which forms the famous egg-shaped mountain peaks depicted in traditional Chinese paintings.

That is why China is home to so many tiankeng. Of the 300 known “heavenly pits” in the world, around 200 are in China. They are mostly distributed in the western half of the country, from Shaanxi province in the northwest to the Guangxi Zhuang Autonomous Region in the southwest.

Guangxi — home to the world-famous karst landscapes that draw millions of visitors to Guilin and Yangshuo each year — is also a global center for tiankeng. The region alone accounts for one-third of the world’s “heavenly pits.”

New tiankeng — even whole groups of them — continue to be discovered on a regular basis. The Hanzhong Tiankeng Cluster — China’s largest known chain of tiankeng — was found in Shaanxi province as recently as 2016.

The first visitors to the Hanzhong cluster spent days exploring the virgin forests at the bottom of the tiankeng, which are connected by a network of caves and underground rivers.

In May 2022, images of another lost subterranean world went viral around the world, after a team of scientists descended into a newly discovered tiankeng in Leye, a county in Guangxi.

Leye was already known to have 30 tiankeng, and it’s remarkable that this latest sinkhole had remained unnoticed for so long. It’s a whopper: almost 200 meters deep and with a volume of 5 million cubic meters.

Inside, the explorers discovered trees towering 40 meters high — part of an ecosystem that had never been disturbed by humans.

Denizens of the deep

Scientists are only just starting to properly survey the biodiversity of China’s tiankeng. Though most research so far has focused on plants, the sinkholes are already known to be home to a large variety of animals.

There are flying squirrels, civets, as well as many species of snakes, frogs, and lizards. The caves inside host dozens of different species of bats, which play vital roles in ecosystems — controlling the insect populations and acting as pollinators.

Then, there are some truly weird fish. Ghost-white, blind cave fish, some with strange protrusions on their heads, lurk in the underground rivers that flow between the tiankeng.

Guangxi has an incredible diversity of cave fish, and many of the species are known to science. Yet, some cave fish species are already considered endangered or vulnerable, especially those that are only found in a single cave system.

Insects are another group of otherworldly-looking inhabitants of tiankeng ecosystems. Giraffaphaenops clarkei — a fierce-looking beetle discovered in Leye County in 2002 — is a good example. Armed with massive serrated jaws, it takes its Latin name from its long giraffe-like “neck,” which allows it to forage under rocks.

New species of spiders and insects continue to be discovered in the tiankeng, and dedicated surveys of China’s sinkhole fauna are sure to add to the list.

But it’s the diversity of plants inside the tiankeng that most excites Chinese scientists. Sheer rock walls, hundreds of meters high, have prevented humans from reaching the bottom of the pits, protecting the forests growing there.

Sheltered inside these virgin forests are a number of plant species that have gone extinct outside the tiankeng. In 2016, biologists exploring a tiankeng cluster in Guangxi discovered a dozen species that had never before been recorded in the region, including a shrub, orchid, lily, rock jasmine, and seven new bryophytes (a group of primitive plants that include mosses).

More recently, in 2021, botanists working in a tiankeng in southwest China’s Yunnan province re-discovered two plant species — Petrocosmea grandiflora and Elaeagnus bambusetorum — that had not been seen in the wild for a century.

Future ‘genetic libraries’

In the future, scientists predict that tiankeng will become even more important. The “heavenly pits” are a distinctive ecosystem in their own right. Inside, the air is cooler and moister than on the surface, and soil nutrients are very abundant.

These environmental conditions already allow unique and little-researched plant, fungal, and microbial communities to flourish. And as global warming accelerates and land surface temperatures rise, scientists expect tiankeng will become refuges for more and more species that may otherwise be driven to extinction by excessive heat.

There are also hopes that tiankeng can help China’s scientists reverse environmental damage that has already been done.

Ecosystem restoration projects — bringing back species of plants and animals — are now gathering pace across China, from the grasslands of the Tibetan Plateau to the mangrove forests of the tropical Hainan Island.

But to be effective, any ecosystem restoration effort needs to use the right combination of native species. This can be difficult, as we may not know which species were originally present before the damage was done.

That’s where tiankeng come in. According to scientists, the sinkholes are ready-made “genetic libraries” for local ecosystems in China’s karst region.

Striking a balance

There’s a danger, however, that many tiankeng will themselves be ruined. Most of the sinkholes are located in remote, poverty-stricken parts of China. And local authorities in these areas are often keen to turn newly discovered tiankeng into revenue-generating tourist attractions.

In Guangxi — a region that only recently eliminated absolute poverty — several tiankeng have already been made accessible to visitors. At Dashiwei, a particularly large sinkhole that was discovered in 1997, a futuristic “Sea of Clouds Celestial Ship” observation platform has been built at a cost of over 2 billion yuan ($276 million).

Visitors to Dashiwei can now walk out over the 65 million-year-old chasm, suspended more than half a kilometer above the pit floor. There aren’t any access routes to the bottom of the tiankeng, which means the local ecosystems are mostly protected for now. But a 2019 study by local authorities in Guangxi warned that “any further development would be considered as (environmental) damage.”

At other sites, the development has been even more rampant. Cotton Tiankeng — a sinkhole in northern Guangxi — has been converted into a four star-rated tourist attraction, with an elevator and walkways leading down to the bottom of the pit. There is even a light show held inside the sinkhole. 

It’s easy to understand why local authorities in these areas are pursuing these projects, which can provide badly needed tourism income. But scientists argue that the potential damage done to China’s biodiversity could be incalculable.

China has taken great strides in strengthening protections for its natural environment in recent years. It is currently setting up the world’s largest system of national parks. Wildlife protection laws have been strengthened, and flagship species including tigers, leopards, and bears are making a comeback.

Now, Chinese scientists are urging the authorities to extend such policies to include China’s “heavenly pits,” which could be one of the country’s greatest environmental assets.

“Tourist activities have created new perils for the tiankeng ecosystem,” one group of scholars wrote in a recent paper. “Conservation efforts should prioritize tiankeng habitats and their rich biodiversity.”

Editor: Dominic Morgan.

(Header image: A heart-shaped sinkhole in Zhenba County, Hanzhong, Shaanxi province. The sinkhole is called “Tianxuan.” From @真实探索税晓洁 on Weibo)