Phong Nha sinkhole discovered: 120 m deep, 2026

In July 2025, we at Jungle Boss and American cavers from the National Speleological Society (NSS) descended a giant Phong Nha sinkhole in Cha Noi-Khe Gat, within Phong Nha-Ke Bang National Park. It is 120 m deep, with an opening about 190 m x 140 m and a floor about 250 m x 200 m.

We first reached the rim with our food and drinking water gone. This time we came with ropes, and lowered ourselves towards the forest below.

The sinkhole is not open to visitors, and there is no tour to it. We found no cave entrance at the bottom.

Finding the Phong Nha sinkhole, one drone flight at a time

A large, unusually dark patch on Google Earth started the search. Kevin Ditamore, a long-standing member of the National Speleological Society (NSS, USA), spotted it while reviewing satellite imagery for geological clues to caves in Phong Nha-Ke Bang.

Kevin brought the images to other American cave experts and Nguyen Van Uy, Deputy Director and Safety Director of Jungle Boss. We arranged drone flights to investigate. The satellite images were indistinct, and the patch lay in difficult terrain: it took three attempts to establish the sinkhole's exact position.

Uy recalled that nobody knew whether any route led to those coordinates. Every cave in our Phong Nha guide has a trail to it; this opening had none.

Drone panorama of the sinkhole from above. Photo: Jungle Boss.

April 2025: reaching the rim with the water gone

We sent the first images to the national park management board. With Cha Noi-Khe Gat identified as the target, we designed an approach around a trail used by forest rangers on their monthly patrols.

On 27 April 2025, we set out with 10 members, including two local forest rangers. We spent five days in the national park. It was the dry season, in an area with almost no large streams, and water became the hardest part of the journey.

We collected what remained in small pools and rationed every drop. On the third day we reached the rim, but our food and drinking water had run out. The wall fell away below us. We had come far enough to see the descent we wanted to make, and now had to turn back.

Steep, sharp rock walls lead down into the giant sinkhole. Photo: Jungle Boss.

July 2025: cat's-ear karst, a rock seep and the ropes

We returned in July with 24 members to descend and measure the whole sinkhole. We had set aside 23 to 29 July 2025 for the expedition, using the route and camps established in April.

The first day took us about 10 km through dense forest. On the second we walked another 11-12 km and camped without a stream or lake nearby. Our only water came from a small seep in a rock face. It yielded just enough for cooking; washing and personal hygiene had to wait.

On the third day, another 3 km brought us across sharp cat's-ear karst to the rim. Among the rocks lay bottles and rusted cans that looked as though they had been there for 20-30 years.

Our technicians and the NSS experts rigged the safety system. We went over the edge one by one, lowering ourselves down the vertical wall with the forest beneath us. On a multi-day cave expedition in Vietnam, the camps and approach are as much a part of the journey as the cave itself. Here, three days of walking had brought us to the start of the rope.

Kevin descends into the giant sinkhole on ropes. Photo: Jungle Boss.

A forest below the rim, shells along the walls

At the bottom we measured an interior wider than the opening above us. The oval mouth was about 190 m x 140 m; the floor extended about 250 m in length and 200 m across, 120 m below the rim. The sinkhole opened out like an inverted bowl, ringed by walls full of cavities and knife-sharp ledges.

“This is one of the largest sinkholes I have ever seen in more than 20 years in the forest,” said Nguyen Van Uy.

Large, old trees filled the floor. The experts believe this primary forest points to a sinkhole that formed millions of years ago, and that a powerful underground river once ran beneath such an extensive collapse.

A primary ecosystem with large old trees occupies the sinkhole. Photo: Jungle Boss.

We found millions of shells, including snail shells, heaped beside the walls, with numerous monkey skeletons among them.

We searched the walls metre by metre for geological signs of an underground passage. A fissure reached 5 m high and 12-15 m deep, without airflow. Landslides had buried the foot of the walls and the former watercourse beneath soil and rock.

Numerous snail shells indicate life within the sinkhole. Photo: Jungle Boss.

“The water here drains through hundreds of tiny holes and cracks, rather than collecting at a single point. We found no opening of any significant size, or any airflow rising from the bottom of this sinkhole,” Kevin said.

NSS and Jungle Boss experts believe a cave system may still exist in the area, with its entrance collapsed by natural processes. At 120 m, our descent was far short of the vertical distances surveyed in the world's deepest caves. What stood out here was the broad, forested floor beneath that oval opening.

Taking the measurements back to the national park

On 27 July, we decided to withdraw early for safety. Back from the forest, we submitted our measurements and a detailed report to the management board of Phong Nha-Ke Bang National Park, part of a UNESCO World Heritage property.

We are proposing further study across Cha Noi-Khe Gat with geology and geomorphology experts, as well as new surveys to look for an approach from outside the sinkhole.

If you want to stand at the bottom of a sinkhole yourself, Kong Collapse is the Jungle Boss expedition built around one. Hang Pygmy takes you into a vast cave for another kind of journey underground.