The Great Passage Of Phong Nha-Ke

For thousands of years the great cave of Hang Son Doong (also known as Mountain River Cave) has remained hidden from humans, hidden by lush green forrest. Situated in Vietnam’s Phong Nha-Ke Bang National Park, near Laos border, Son Doong remained secret until the spring 2009, when the first expedition expedition of the cave took place. Lead by Howard and Deb Limbert (who have lead 12 explorations to Vietnam’s limestone caves in the past) the expedition discovered the biggest cave in the world, taking over it’s predecessor Deer Cave (or Gua Payau) in Borneo.

Though the Vietnamese natives were aware of the cave, grave superstition and great fear kept them from exploring it. It wasn’t until Ho Khanh, a native of a nearby village, found the cave. When he lost his father as a child, Khanh was left to fend for himself and foraged through the deep, thick forrest for food. He discovered the caves when he had to find shelter from rain and the bombings which devastated Vietnam during the Sino-Vietnamese conflicts.

man climbing in cave

Down in the darkness lies a barren landscape of rock and mud. The acidic water and regular flooding has prevented the development of life, leaving the majority of the cave lifeless and empty. However, some of the dry, side passages have harboured life. In the desolate environment Zoologist Dr. Anette Becher discovered a new species of woodlouse, one unique to Son Doong. Although evolution in mammals is highly unlikely to occur in a small stretch of time it’s much more likely to appear in insects. These woodlouse can go through two or three generations a year, speeding up the process immensely. The woodlouse is pigment-less, appearing white as opposed to the usual grey or black colouring we’re accustomed to. The lack of sunlight makes their grey bodies useless so they pigment has slowly faded into nothing over the years. It is due to the isolation of the cave, the darkness and the primitive life, that these woodlouse have evolved into an entirely new strand.

woman above cave

With his help, the Limberts managed to find Son Dong, though it took three attempts. The Limbets had no idea that they had just discovered the biggest cave in the world.

Unfortunately, after half a week of being inside the cave, the Limbert’s expedition was cut short when they encountered a 235ft wall of muddy calcite. Another team was sent in, but were also unable to complete the passage due to high water levels. Finally, in 2010, an expedition team managed to reach the end and defeat the monstrosity dubbed ‘The Great Wall of Vietnam.’

The crew, again, lead by Howard Limbert, made up of seven cavers, a few expert scientists and porters entered the cave for a third time. This time they were joined by journalist Mark Jenkins, who documented their journey for National Geographic.


However, this isn’t the only evolution that has taken place in Son Doong. Within the cave lives a huge chamber with a collapsed roof; a collapsed doline. Within the camber lies a unique jungle that creeps some way into the passages of the cave. The roof collapse lets the ingredients for life in: water and sunlight. Seeds get dropped by animals or birds and start to grow although they are trapped to the confines of the cave. Like the dry passages Dr Becher thought that the the chamber would contain some new species of life but was unfortunately disappointed. Nonetheless she did find something equally as interesting. The plants in the doline are the same as those in the surrounding jungle, but show extreme adaptation also known as phenotypic plasticity. The plants may be of the same species, but they have adapted to survive.

Most notably of these adaptations, the trees, which are usually evergreen, have become deciduous (they shed their leaves) and have narrow trunks and sparse foliage due to the limited sunlight and water. This fantastic discovery shows the versatility and durability of the plants, their phenotypic plasticity has allowed them to survive when other plants of the same species would die.

But the ecosystem of Son Doong isn’t the only unique thing about it.

Throughout the cave runs the Rao Thuong River. How Son Doong managed to get to its massive size has remained a mystery for many years and Geolgist Dr. Darryl Granger managed to solve it. He originally thought the cause was highly acidic water. The tropical rainstorms of Vietnam cause an abundance of water which seeps through the soil, picking up CO2 which makes it acidic. In turn the limestone, which make up the cave walls, dissolves rapidly. Surprisingly Dr. Granger’s theory was proven wrong. For every gallon in the water there was 8oz of rock, which is surprisingly little when considering the size of the cave. The water was tested for acidity; it was found to have a pH of 7.5. On the pH scale the lower the number the more acidic it is and 7 is considered neutral, meaning the water isn’t acidic at all.

Dr. Granger looked for other causes. His second theory was the  answer lay in the speed of the water. Within the walls lie scallops (little dips in the rock’s surface) and the smaller the scallops the faster the water, but the scallops were found to be of average size. The water wasn’t flowing any faster than the average river which ruled out another theory.

Time was running out when Dr. Granger fortunately noticed a fault line following the cave surface on a map of Son Doong, finally leading him to the answer of this riddle. Firstly Son Doong runs in a straight line, from north to south. This is unusual in cave formations and all along this straight line lay the fault line. It became apparent that a river once ran along the surface of the of Phong Nha-Ke Bang National Park. The water flowed down the fault line and found an exit and the river eroded the rock along the crack. The narrow fault line also restricts the cave width, preventing it from collapsing in on itself and allowing it to grow to the massive size it is today.

inside the cave

But how big is the cave? And how can it be measured accurately? In the past cave measuring equipment was primitive. Most of it was guess work and estimation. But today lasers are used for accurate, and even exact, readings. Son Doong is 656ft wide, 492ft high, and approximately 5 miles long. Some of the caverns within the cave are big enough to fit entire city streets within them. Comparatively, it’s forerunner, Deer Cave is only 295ft wide, 328 meters high and a mile long clearing marking Son Doong as the clear winner. However, Deer Cave can still claim the largest cave chamber. It’s Garden of Eden doline is three times bigger than London’s Wembely Stadium and experts wonder if they’ll ever find a cavern bigger.

graph comparing caves

Having such a huge wonder lying beneath the surface of the Earth opens our eyes to what else could be out there. Though the human race prides itself on travelling all four corners of the globe there are many things we still haven’t found. The Amazon is a market place of undiscovered species, we have only explored 5-7% of the ocean floor and more recently caves in China have been discovered to have their own weather. This begs the question: what else lies out there completely undiscovered?

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