Son Doong, the largest cave on Earth, is a colossal underground world with a self-sustaining ecosystem and unique climate. Deep beneath the surface, clouds form within the cave, and it harbors an untouched prehistoric rainforest and a vast underground river. Home to over 250 endemic plant and animal species, including transparent fish and colorful monkeys, Son Doong recently revealed seven new species unique to its depths. Spanning 9 km with 38.5 million cubic meters of space, its passages are so vast a 40-story skyscraper could fit inside. Towering stalagmites rise like ancient city spires, dwarfing visitors. Remarkably, the cave contains phytokarst rocks that move toward sunlight like semi-living organisms, alongside rare cave pearls and other unique formations. Unknown to the world for 3 million years, Son Doong was fully explored in 2009 in Vietnam. To protect its fragile environment, individual access is restricted, with expeditions led solely by Oxalis, the authorized agency.
Our journey to Son Doong began with a 9-hour trek through dense jungle, followed by passage through Hang En, the world’s third-largest cave and gateway to Son Doong. Thorough online research prepared us for the challenges ahead. Inside, we reached a beach-like area carved over centuries by nearby rivers. The cave’s ceiling soars 145 meters, its width spans 200 meters, and during flood season, much of the space submerges, with water levels rising dramatically. A stunning lake tempted us for a quick dip. At dusk, swiftlet birds’ echoes filled the cave; unable to take off from the ground due to weak legs, they leap from heights to fly, and a grounded bird faces certain death. At sunrise, we ventured deeper, comparing Hang En to a neighborhood and Son Doong to a sprawling city.
Navigating the dark, we encountered jumping crickets and ground covered in centuries-old bat guano. Above, countless bats slept, some awake and defecating, making looking up unwise. Limestone steps led to travertine-like formations and water trickling from both sides. The 450-million-year-old limestone, formed from ancient sea creatures’ shells and skeletons, was shaped by acid rain dissolving calcium over millennia, creating the cave. A final climb brought us to Son Doong’s grand entrance, where headlamps illuminated the Hope and Vision Passage. The cave’s mineral-rich water formed rimstone pools, and a cricket greeted us. Sparkling white stalagmites, bronze stalactites resembling frozen waterfalls, and broken formations hinted at iron and tectonic activity. A fast-moving river posed a deadly risk, fed by heavy rains carrying soil and plant stems, fostering mushrooms in the moist darkness.
In the Hope and Vision Passage, we stood beside the world’s tallest stalagmite, 80 meters high, formed over 800,000 years in a chamber 170 meters tall and 130 meters wide—large enough for a double-decker plane. Cloudy white patches revealed spiderwebs trapping insects, and sand towers formed by water erosion resembled a miniature city. At our campsite, 3 km in, we glimpsed the first doline, a massive collapse letting in light. Mist and clouds formed due to temperature differences, and beyond lay the “Watch Out for Dinosaurs” area, to be explored the next day. Crawling through narrow passages, we reached the first doline, where cave worms’ bioluminescent threads trapped insects. The 400-million-year-old limestone held fossils, and a 140-meter-high wall marked the doline, formed by a collapsed roof eroded by the river. Algae and mosses thrived in the light.
The second doline, far larger, revealed a dense underground rainforest. A collapsed roof 252 meters above allowed sunlight to nurture a unique ecosystem. Fertile soil, rich with bat guano, supported over 200 plant and 60 animal species, including monkeys—a rarity for caves. Tall, thin trees reached 40–50 meters to capture sunlight, while monkeys broke snails with stones for food. On our fourth day, we navigated a water-filled passage to the Great Wall of Vietnam, an 80-meter karst wall. Stalagmites resembled sculptures, and eyeless fish and crickets, adapted to darkness, swam in pools. Cave pearls, some tennis-ball-sized, formed over centuries. Deer bones, 500 years old, hinted at past life. The Passandale Passage, now a jade-green lake, led to the wall, with the cave’s exit visible 500 meters away.
A lingering mystery was a fossil’s identity. Using Opera browser, we accessed Son Doong’s website, where Aria, an AI tool, highlighted article key points. Uploading the fossil’s image, Aria identified it as Tetracoralia (rugosa), an extinct carnivorous coral from 300 million years ago with tentacles, growing up to 10 cm. Opera’s tab islands and split-screen features aided research, while Aria’s image generation visualized the creature. Opera’s tools, including a floating music player and customizable themes, enhanced our workflow. Son Doong, a testament to nature’s grandeur, left us in awe of its secrets and beauty.
