A lost rainforest in the worlds biggest cave - How to Grow a Planet - BBC Two

I've directed many of the Iain Stewart series and we always try to showcase something that’s never been seen before. For How Earth Made Us we faced incredible humidity and heat to film Cueva de los Cristales in Mexico. This time we were in Vietnam, facing a whole new set of challenges to film Hang son Doong - the biggest caves in the world. Just getting our team and equipment into the caves was an immense undertaking...

Iain Stewart on route to Han Son Doong

The caves are in central Vietnam near a town called Dong Hoi – which was frontier country during the Vietnam war. This is where the fighting was at its most intense. It was sobering to think that the town in which we landed had been obliterated 30 or so years ago. When it came to our trek through the jungle, we strictly followed our guides and porters to be sure that we didn’t encounter any unexploded mines. As if that wasn't dangerous enough, next we had to scale precipitous sharp rocks half way up a mountain. Suddenly we saw the tell-tale clouds of steam rising up out of the jungle – the mouth of the cave. The clouds are formed by the cool air of the cave causing the thick moisture-laden forest air to condense. 

But the hard work was only just beginning! From the foreboding cave entrance we vertically abseiled 100 metres into the pitch black,  slowly passing calcified flowstones called the Great Wall of Vietnam.

 Hang Song Doong, Calcite flowstones coat the Great Wall of Vietnam. (Photo Credit: ©Simon Reay)

 Slowly abseiling down the wall of Vietnam (© Carsten Peter/National Geographic)

Deep in mud at the bottom of the wall of Vietnam (© Carsten Peter/National Geographic)

The Garden of Edam

After hours of trekking through the dark and dirt we eventually saw a glimpse of what would be the fruits of our labour. Hidden at the heart of the cave system, a vast cavern has collapsed – and in a pool of light is a rainforest - 'The Garden of Edam'. It was completely astonishing to turn the corner and see green in the distance! 

In this oasis of green everything stretches upwards towards the light – trees are tall and spindly, leaves turn and face one direction… and even here, on the rainforest floor – are flowers. Streptocarpus, orchids, banana plants, and it's rich with animal life – insects and birds. It's a microcosm of the rainforest above.

Hang Song Doong, Son Trach, Bo Trach District, Vietnam. (©Simon Reay)

 Professor Iain Stewart in The Garden of Edam (Photo: Fraser Rice)

 Hang Song Doong, Vietnam. (©Simon Reay)

Rare cave pearls fill dried-out terrace pools near the Garden of Edam. These stone spheres formed drip by drip over the centuries as calcite crystals left behind by water layered themselves around grains of sand, enlarging over time. (Photo Credit: © Carsten Peter/National Geographic)

Working in the mud

The entrance to the Garden of Edam was to be our base for 4 nights and it was to be a real endurance exercise. The floor of the cave is covered in very fine and slightly caustic dust which acts more like cement. It wasn't long before everything got caked in mud and because it was the dry season there was no water to wash in. Each day started with us plastering our feet in cream and powder to prevent the onset of trenchfoot. And of course the filming equipment had to be kept meticulously clean.

The biggest caves in the world

The rainforest was the pinacle of what we wanted to film, but the Hang Son Doong caves were awe-inspiring in themselves, the very largest caverns in the world. Apparently, St Paul's Cathedral could fit comfortably inside. I wanted to show the scale of these caves – to illustrate the erosive force of water produced by the rainforest above. You can actually see vast distances inside with the naked eye – up to a mile or more if the light is right. The challenge was trying to make that visible on camera.

Nigel with a block of ice having just emerged

One of the ways we tried to capture the sense of awe was to make the camera float through the immense space, flying past huge rocks or through tall spindly trees. We used a cable dolly – a long tension wire with a remote controlled set of wheels that glides along with the camera slung underneath. It took almost an hour to cross from one side of the Garden of Edam to another – but we managed to do 3 different cable dolly positions on our first day of filming plus all the pieces to camera – very satisfying shooting!

Next job was a top-shot of the forest and Iain. For cameraman Keith Partridge, it meant climbing 100m vertically up a free-floating rope – carrying his camera on his back!

We were very privileged to be able to film in these caves and the Vietnamese authorities kindly granted us access – we’re the first all British film crew to go inside. And of course we were indebted to our guides - a 5-strong British team led by Howard and Deb Limbert who have pioneered the exploration of the caves in central Vietnam over the last 20 years. We couldn’t have done it without you guys! Thank you!


Watch a clip from the film