On the first day of their expedition, the explorers crossed a stream near the mountain’s foot to reach Doong Village and then went straight forward to En (Swallow) Cave – the first stop on their journey in Quang Binh Province.

They were on a trip to implement the Son Doong 360 project, expected to show exclusive 360-degree images of Son Doong on the online edition of NatGeo.

Son Doong 360 was initiated by Swedish journalist Martin Edström, 27, who graduated from Stockholm Unviersity in 2010 and is now working as a photographer for UNDP and the Kontinent Agency of Sweden.

The cave – formed two to five million years ago by river water erosion of the limestone underneath – was discovered in 1990 but only announced publicly six years ago thanks to the assistance of British explorer Howard Limbert.

The number of caves in Phong Nha-Ke Bang National Park, home to Son Doong, remains unknown.

Though the path to En Cave was simple, the explorers had to cross 20 sections of streams with water reaching their knees.

The second working day started at En Cave, where the team of explorers spent their first night. In the early morning, team members Sebastian and Erik came to collect images from a GoPro camera automatically set to capture an image every 30 seconds.

As many as 270 photographs were ‘linked’ by software to create a film strip running from the first moment when the cave was lit by sun beams to the time the sun shone brightly through the cave’s mouth, creating a column of light illuminating the lake inside En Cave.

En Cave was formed thanks to its two mouths and the brooks in the area millions of years ago.

Initially, water poured into the upper mouth. Erosion and geological changes under limestone mountains formed the current shape of the cave. Erosion over such a long time period created the lower mouth.

Now, the upper mouth only sees water when floodwaters rise up over ten meters.

English explorer Howard Limbert, who carried out the exploration of and publicly announced the existence of Son Doong Cave in 2009, said locals in Phong Nha have invaded the cave and climbed sheer cliffs to collect bird’s nests and catch swallows.

Several years ago, two men fell from the cliff and died immediately.

The path from En Cave to Son Doong Cave runs along the Rao Thuong River. This is one of the rivers that flow into holes at the feet of limestone mountains to create caves.

In the dry season, it is easy to cross the river since the water only reaches the waist. But it becomes dangerous in the rainy season, when tours of Son Doong pause.

Halfway from En to Son Doong, Limbert called the other team members and pointed toward a cave on the left side. This is Khe Ri Cave, which is recognized as the cave containing the longest river in the world.

Khe Ri Cave is 19km long and reaches the border of Vietnam and Laos.

Next to Khe Ri is the mouth of another cave which had not been discovered before.

Limbert said that he has discovered several previously unknown caves in Phong Nha-Ke Bang National Park.

Toothpaste to protect the environment

Explorers around the world place great importance on protecting the environment.

They do not bring soap on their trips because the chemicals used in it may cause harm to trees and water.

In Australia and other nations, explorers can find a special kind of toothpaste for exploration that can be swallowed after brushing teeth, instead of spitting it out.

Tuoi Tre (Youth) newspaper journalists could not find this product in Vietnam and had to use the traditional product – salt, which is good for gargling after use.

It should be noted that the shapes and structures created inside caves under limestone mountains are vulnerable to impacts from the environment and human beings.

Human beings have fallen into a vicious circle. Developed nations try to limit emissions to protect the environment, while developing nations are willing to do everything for economic development.

When they become richer, they will spend time regretting the environmental loss that money cannot compensate for.