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Divers discover tunnel inside world’s biggest cave linking it to another big cave

Vanessa Brown, May 21, 2019 7:00PM

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Inside Son Doong cave, Vietnam. You can see the rows of tents set up on the right. Picture: iStock media_cameraInside Son Doong cave, Vietnam. You can see the rows of tents set up on the right. Picture: iStock


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Explorers have discovered a massive tunnel inside the world’s largest cave, linking it to another huge cave.

The divers who helped save the Wild Boars boys’ soccer team in Thailand last year led the expedition.

Their incredible discovery has been described as like finding an extra lump on the top of Mount Everest, making it 1km higher.

The three UK divers found the hidden waterway inside Son Doong, which is the world’s largest cave. Son Doong is located in central Vietnam in Phong Nha-Ke Bang National Park.

Son Doong was only discovered by accident in 1990 and first explored in 2009. It had never been dived before this expedition.

The tunnel the divers found connects Son Doong (meaning Mountain River Cave) with another huge cave called Hang Thung.

“It would be like someone found a lump on top of Mount Everest, making it another 1000m higher,” Howard Limbert, one of the cave experts who helped organise the dive, told CNN.

“Any cave in the world will be able to fit comfortably inside Son Doong when it’s connected. It’s just outrageous* in size.”

Son Doong was opened to just a few tourists a year in 2013 through a single tour operator, Oxalis.

VIDEO: Inside Son Doong cave

Cave explorers find underground river tunnel

The Son Doong cave is almost 5km long and is thought to be relatively* newly formed — perhaps only three million years old. The mountains the cave is in are hundreds of millions of years old but a big river, called Rao Thuong, has carved out tunnels and caves in the limestone* under the mountain.

The cave is more than 5km long. Picture: istock media_cameraThe cave is more than 5km long and was formed by water rushing through the mountains above it. Picture: istock

Mr Limbert said the divers — Jason Mallinson, Rick Stanton and Chris Jewell — were invited to the cave by expedition company Oxalis after doing such “an amazing job” rescuing the trapped Wild Boars boys’ soccer team in Thailand.

“We invited them on a trip to Son Doong to thank them for their great effort,” Mr Limbert said.

“They wanted to do something interesting during the trip, so we came up with this idea of diving Son Doong, which had never been done before.”

The divers ventured in to unexplored parts of the Son Doong cave in Vietnam. Picture: AFP media_cameraThe divers ventured into unexplored parts of the Son Doong cave in Vietnam. Picture: AFP

Mr Limbert said when the divers reached a depth of 78m, they were able to measure some of the uncharted tunnels within the cave.

“Now that we know how deep it is, we’ll bring the special gases (oxygen-helium mixture) with us next time to enable long, deep dives,” he said.

“I think it’s incredible that something as important as the world’s largest cave is still being explored and better understood.

“No one had ever set foot inside Son Doong until 2009 … and this latest discovery shows there are still an awful lot of things to uncover on this planet. It’s really exciting.”

Phong Nha Ke Bang National Park, Vietnam media_cameraSon Doong cave, the world’s largest cave at the UNESCO World Heritage Site of Phong Nha-Ke Bang National Park. Picture: iStock

Local man Ho Khanh was out hunting in the forests of central Vietnam in 1990 or 1991 when he found an opening in a limestone cliff. Clouds were coming out of the opening and he could hear the sound of a river rushing.

After that day, Ho Kanh and others couldn’t find the cave opening despite many years of searching.

He stumbled across it again in 2008, noted where it was and took others back to see it. It was first explored in 2009 and Ho Kanh is now a guide taking expeditions into the cave.

Spelunker: a person who explores caves; from the Latin* word spelunk, meaning cave

An expedition base camp inside Son Doong. Picture:istock media_cameraAn expedition base camp inside Son Doong. Picture:istock

There are likely to be many undiscovered caves all over the world. Big caves we know about include:

  • Son Doong, Vietnam: 8.8km long; some spaces big enough to hold a 40-storey building; biggest (by volume) known cave system in the world
  • Mammoth Cave, Kentucky, US: 587km explored so far; longest known cave system in the world
  • Mulu Caves, Borneo, Malaysia: largest known cave chamber* by surface area; could hold 40 747 aeroplanes
  • Sistema Sac Actun, Mexico: second-longest known cave system in the world; 209km have been explored so far
  • Krubera Cave, Georgia: 2.2km deep; deepest known cave in the world



  • outrageous: bold, unusual and shocking
  • relatively: when compared to something else
  • limestone: a soft type of rock made from sediment settling
  • Latin: language used by ancient Romans
  • chamber: a large room or space


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  1. Why is this an important find?
  2. Why were the three divers invited to explore Son Doong?
  3. Who first found Son Doong cave?
  4. What does spelunker mean?
  5. What and where is the deepest known cave in the world?


1. Spelunker!
Looking at the video and pictures of the Son Doong cave, imagine you are in charge of your own expedition!

Just a single tour operator, Oxalis, has access to this amazing cave system, and they have invited you to plan and organise a charity event. You have premium exclusive access for one day and night only.

How would you use this amazing resource to plan and organise a once-in-a-lifetime opportunity for a group of people, which will also to raise money for a charity of your choice.

In your planning stage, complete the following questions and outline your ideas to use the cave system.

  • Name of event
  • Date planned
  • Timing of event
  • Charity of choice
  • Price of experience
  • Number of people limited to
  • Outline of experience to be offered

Time: allow 25 minutes to complete this activity
Curriculum Links: English, Critical and Creative Thinking

2. Extension
Create a travel brochure/invitation to the charity event planned above for the Son Doong cave. You can do it on coloured card or using a computer program. Make it look appealing, interesting, use the amazing photos and outline all the information about your charity event/expedition.

Time: allow 30 minutes to complete this activity
Curriculum Links: English, Digital Technologies, Critical and Creative Thinking

After reading the article, with a partner, highlight as many connectives as you can find in pink. Discuss if these are being used as conjunctions, or to join ideas and create flow.

HAVE YOUR SAY: Would you like to explore unknown caves like these divers?
No one-word answers. Use full sentences to explain your thinking. No comments will be published until approved by editors.

Extra Reading in geography