A Crack In The Mountain is a feature length documentary film which tells the story of the world’s largest cave.

Hang Son Doong which translates as “mountain river cave”, is located in the Phong Nha-Ke Bang National Park in Quang Binh Province in Vietnam. The national park is also a UNESCO world heritage site.

First found in 1990 by a local farmer, it wasn’t explored until 2009 when a British expedition team rigged ropes and descended down.

Many people have described Son Doong as a lost world. It has its own lake, jungle and even a unique weather system.

In 2014 plans were announced to build a cable car into Son Doong.

With many arguing that this would destroy the cave’s delicate eco-system and the local community divided over the benefits this development would bring, the film follows those caught up in the unfolding events.

The documentary also uses the narrative as a lens through which to investigate related themes such as the challenges of modern day exploration, environmental conservation & sustainability, and the perils of operating as an activist in a country such as Vietnam where freedom of speech is severely curtailed.

"Natural wonders like...Son Doong Cave have to be preserved for our children and our grandchildren."

- Barack Obama, Former US President

"Discovering Son Doong is like finding a new Mt Everest. There's nothing else like it in the world."

- Howard Limbert, Leader Of The First Expedition To Sơn Đoòng

"Son Doong is like a hidden treasure. It's been forming for millions of years and remains in pristine condition. This is what makes it so special."

- Huong Nguyen Thien Le, Co-Founder Save Son Doong

How big is Son Doong Cave?

Hang Son Doong is the biggest cave in the world by total volume.

To put that in context, Mammoth Cave in Kentucky, USA is 640km long and is the longest cave in the world. Sơn Đoòng is just over 9km long and is big enough to hold all of Mammoth Cave.

Son Doong also has the biggest passage dimensions of any cave in the world. At points, its up to 160m wide and 160m high. This means a Boeing 747 could fly through it.

In the chamber containing the first skylight, the cave passage is a little over 400m high. This is the equivalent of a hundred story building.

How did it get so big?

This is due to the amount of rain which falls in North-Central Vietnam as well as the purity of the limestone rock in the Phong Nha-Kẻ Bàng National Park.

Pure limestone dissolves very easily but is also very strong. This means that after the rock has been eroded away, it can still support its own weight.

Heavy storms and an intense monsoon season are a regular occurrence which brings a lot of rainfall. Over millions of years this has dissolved large sways of limestone leaving in its place Hang Son Doong.

Is its size the only thing which is special?

Far from it. Many would argue that the world inside Hang Son Doong is more impressive than its size.

The roof collapse which formed the cave’s first skylight (known as a doline) created a natural dam. This means that two thirds of Son Doong has not been damaged by river erosion for over half a million years. All the cave’s beautiful formations are perfectly preserved.

Son Doong contains it own unique eco-system. Light streaming in through the second doline has allowed a large jungle to grow and thrive.

At certain points in the year a pristine lake appears in the final part of the cave system. Rolling clouds are clearly visible at the first campsite.

To date, seven new species have been discovered inside the cave. This number continues to grow.

  1. Son Doong Cave is formed

    A trickle of water enters a fissure in a large bed of limestone rock.

    The water dissolves the calcium carbonate and makes the hole bigger. A river begins to flow through the widening space increasing the size still further.

    There is a collapse and a large skylight is formed re-directing the river down another route.

    The now empty cave passage begins filling up with calcite formations.

    Hang Son Doong is born.

  2. The Vietnam War

    An American pilot on a bombing raid spots the first skylight opening from above and drops a bomb down it.

    Remnance of this is still visible today.

    There is no record of him reporting the sighting.

  3. The entrance to the cave is discovered

    Ho Khanh, a local farmer from the nearby village of Phong Nha, is out in the jungle hunting and collecting rare wood to provide for his family.

    He gets caught in a storm and seeks shelter under a nearby cliff.

    He notices a small opening in the rock from which clouds are pouring out and a river can be heard deep below.

    He doesn’t go in.

  4. British explorers enter Son Doong for the first time

    A team of six British cavers rig ropes and are the first to enter Son Doong.

    Over the course of five days, they explore it, going deeper and deeper into the cave system.

  5. Son Doong opens for small group treking tours

    The Vietnamese government grants permission to Oxalis, an adventure tour company, to run multi day treking tours into Son Doong for small groups of no more than ten people.

    The tours prove very popular. There are only a limited number of spaces and demand for these greatly outstrips supply.

  6. Plans to build a cable car are announced

    Government officials from Quang Bình Province announce plans to build a $212 million cable car system inside Son Doong with the intention of opening up the cave to more visitors.

    According to one local official, the cable car would carry up to 1000 visitors per hour.

  7. The Save Son Doong movement is formed

    In response to the cable car threat, a group of young Vietnamese activists come together to try and stop the development.

    Their principle aim is to raise awareness about the destruction the cable car will bring as well as explaining to people the scientific values of Son Doong and why it needs to be preserved.

meet us

production team

Alastair Evans

Producer, Director, Cinematographer

Saso Puckovski

Sound Design

Ryan Deboodt