Four days in the world's largest cave

Hanoi designer Ha Duy, 33, and his friends began their 10 km journey at Hang En (Swallow Cave) in Quang Binh Province - where many swallows can be found nesting. After a day here, the group exited the cave at the back, crossed a stream and passed a segment of a dense forest - a task made possible thanks to experienced guides that accompanied them.

After nearly a day, the explorers reached the entrance of Son Doong, the world's largest natural cave, yet to be exploited by excessive tourism.

The most "goosebump-inducing" experience for Duy was when he swung himself down into the cave. "It was too slippery and dangerous," he said. Duy had to take a good tight grip of the rope and learn to balance his shoulders before slowly sliding down. "The feeling of walking on a steep rock wall with underneath me nothing but darkness was really scary."

After descending into the cave, the group relied on their headlamps to navigate fast-flowing streams via suspended bridges, a journey of four hours. Duy said he was impressed by the beauty of stalactites he described as "beyond [my] imagination."

The group camped by a sinkhole inside the cave on their second night. "This is the most beautiful place in the journey," he said.

They began the third day of exploration with a lot of climbing and crawling to exit the first sinkhole.

The challenging journey paid off though - as they got to see how nature crafted an exquisite stalactite hole, and other unusually beautiful shaped rock formations.

The team reached the second sinkhole in the afternoon of the third day. They got to swim in a clear stream, witness the cave's largest stalactite cluster, alongside exotic animals like eyeless white fish and crickets with super long antennae.

"The night at the second sinkhole was a new experience because the echoing effect inside the cave made hearing any small sounds possible."

Duy described the last day of the journey as the most challenging, with the team worn out by the physical activities of the previous days. They finally found an exit while proceeding by row boat. "The lake is very large, when looking up, I could not see the ceiling of the cave even though the flashlight [on my head] was very bright."

Duy stands next to what is called Buc Tuong Vietnam (Vietnam Wall), which is as high as a 30-story building.

The designer said it took him more than two months to prepare himself physically for the trip, including taking climbing lessons. "I am a scared person, afraid of heights and dark spaces, but I still like to take risks. When I saw pictures of Son Doong online, I was very excited and wanted to register," he said.

Duy also had to buy trekking shoes, clothes appropriate for the diverse climate inside the cave, masks, medicine, and some energy food.

The team consisted of 10 people including five Vietnamese experts and one foreign expert on the terrain. A group of 25 people went ahead of the team to bring certain items needed for the trip to designated destinations, on top of preparing food and tents for the explorers.

Duy said the trip helped him gain valuable experiences in the wild, especially with other explorers, and overcome his fear of heights and darkness.

Son Doong is estimated to be between 400 and 450 million years old, but it was only "discovered" in 2009. The cave, which has a total length of 9 km, opened to tourists in 2013, four years after members of the British Cave Research Association finished their exploration. Oxalis is the only private company licensed to explore and conduct tours of Son Doong.