630 feet below the Earth in China, an ancient forest blooms at the bottom of a sinkhole

In May, a team of spelunkers rappelled into the dark heart of a 630-foot-deep sinkhole in China’s Guangxi region. Near the southern border, Guangxi is home to a landscape of serrated, mountainous rock riveted by cave tunnels, slicing through pools of jade-green water and a lush thick of trees.

It is among China’s most beautiful terrain, marred by 30 giant, sunken swaths of earth—tiankeng, or “heavenly pits,” in Mandarin.

But this newly discovered sinkhole—deep enough to swallow the U.S.’s tallest national monument, the 630-foot-tall Gateway Arch in St. Louis—houses an even more majestic world. At its bottom, the expeditionists found a flourishing prehistoric forest, with 131-foot-tall trees, branching skyward to a faraway circle of sunlight. The growth of the forest floor, meanwhile, could bury a person up to the shoulder.

As described by Chinese news outlets, it sounds like a near-mythical realm—a fantasy of Narnia, or the kingdom of Middle Earth—and its promise for biologists and geologists is no less thrilling. Scientists believe the forest could harbor previously unknown plant or animal species, as sinkholes can offer an oasis for life to bloom. Such “ancient” or “primitive” ecosystems have never been disturbed by humans. And they are natural jewels for study, offering a glance at what our planet might’ve looked like in primeval times, devoid of humankind’s intrusion.

“I wouldn’t be surprised to know that there are species found in these caves that have never been reported or described by science until now,” Chen Lixin, who led the trek through the sinkhole’s forest, told China’s Xinhua news.

Guangxi is one of the world’s richest treasure troves of karst topography—found mostly in China, Mexico, and Papua New Guinea—where dramatic landscapes are formed by eroding underground bedrock. In such climates, rainwater runs first through soil—becoming gradually more acidic as it saps carbon dioxide from the earth—and then flows through cracks in the bedrock, hollowing it away into sprawling webs of limestone chambers, pillars, bridges, and tunnels. If the chambers grow large enough, they collapse inward and a sinkhole is born.

According to NASA, 13% of China is karst topography, including the world’s largest sinkhole, Xiaozhai Tiankeng, at 2,172 feet deep in Chongqing. Set upon a massive underground river, it also houses a robust forest ecosystem connected to a network of caves, much like Guangxi’s unnamed site.

Scientists now hope to illuminate its untold secrets, hidden in the tiny universes of heaven’s Earthly craters.