Giant sinkholes discovered containing untouched ancient forests

Most of us associate sinkholes with frightening abysses, but in China, they’re considered “heavenly” for good reason.

Far from being mouths of doom, a number of these have been discovered containing pristine ancient forests.

Most recently, in August last year, cave explorers stumbled upon one of these giant holes, which experts said likely contained species of small animals that were unknown to science.

This extraordinary hole reportedly measures a whopping 300 metres (1,000 feet) in length, 150 metres (500 feet) in width and is more than 190 metres (630 feet deep).

It is located in China’s southwestern karst landscape and is now one of 30 such holes known as “tiankeng” – which translates as “heavenly pits” – in the region.

The reason for the trenches’ otherworldly name is that they are some of the last-remaining natural refuges for ancient forests and are home to species unseen anywhere else in the world.

Indeed, describing his own encounter with a tiankeng back in 2016, researcher Tang Jianmin wrote: “The scene down there was stunning: an underground pristine forest with no trace of human activities, with 40 metre-high ancient trees and a group of endangered wild plants from the times of dinosaurs.”

Given their depth, very little light reaches the bottom of the tiankeng, and yet, they harbour an abundance of plants.

For years it remained unclear how this was possible, but a new study, published on 20 July, has revealed how a number of species have been able to thrive in their depths.

The researchers found that nettles, ferns and other plants that live inside them thrive off of abundant stocks of nitrogen, phosphorus, potassium, calcium and magnesium.

These elements limit plant growth in other environments, where they are scarce, but because they are so bountiful in the tiankeng, the plants have evolved to devour them, enabling them to grow tall and benefit from the trickles of sunlight that reach them.

"Plants can adapt to adverse environments by adjusting their nutrient content," the study’s authors wrote, according to a translation by Live Science.

"Due to the towering cliffs and steep terrain of the tiankeng, it has been less disturbed by human activities.”

The deep holes host plants that favour moisture and shade, including species unique to the region, the researchers add. These include modern karst forest plants, such as the Nepali hog plum (Choerospondias axillaris) and the Chinese rain bell (Strobilanthes cusia).

So how do these tiankeng and their forests form?

These remarkable sinkholes emerge when the roof of an underground chamber enlarges and collapses, according to Jianmin.

Some 30 per cent of the world’s tiankeng are found in China’s Guangxi Zhuang Autonomous region among its World Heritage listed karst formations.

Karst landscapes are shaped by the erosion of soluble carbonate rocks such as limestone, creating features such as sinkholes, caves and underground rivers.

The South China Karst are a UNESCO World Heritage site(Ko Hon Chiu Vincent via UNESCO)

In a piece for The UNESCO Courier, Jianmin further explains the formation of these sinkholes, writing: “Karst tiankeng is a relatively closed environment with high humidity, low temperature and a high concentration of negative oxygen ions.

“Its bottom is connected with underground water flow, nurturing a lush green oasis all year round.

“Primitive ecosystems with unique animal, plant and microbial resources are formed there.”

In total, 300 tiankeng have been discovered across the globe, with China home to two-thirds of them. They have also been found in Papua New Guinea, Malaysia and Madagascar, Slovenia, Croatia, Italy and Spain, among other countries.

Why are they important?

More than just a jaw-dropping natural phenomenon, these sinkholes are important ecological havens.

Jianmin noted that he and fellow researchers have discovered many rare and endemic plants in the tiankeng area, cementing its status as a “refuge for endangered plants and primitive forest ecosystems”.

“There is also a good chance that previously unknown species will be discovered in these environments,” he added.

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