Tropical sun getting to you? Explore the secret world beneath our feet in Gunung Mulu National Park.

Photography Chris Howe

The sun is barely up and I’m sitting in the café at Gunung Mulu National Park, a pristine equatorial rainforest in Sarawak, staring down a plate of spicy noodles and planning for the day ahead.

I am here as part of a British group of caving specialists and scientific experts to explore and map the area’s caves. But they are not just any caves; they are the world’s largest. The floor of Sarawak Chamber, the largest known underground chamber in the world, could fit an incredible 400 of the ATR 72 aircraft operated by MASwings, the regional carrier of Malaysia Airlines that serves the park, with plenty of room to spare.

The Racer Cave is named after the non-poisonous racer snake.

This is an expedition I’ve dreamed about for a long time – the caves of Mulu are renowned as much for their exquisite calcite formations and breathtaking streamways as their superlative dimensions. In a single trip, we might expect to stumble across anything from armoured crickets to beautiful blue-and-grey racer snakes, athletic centipedes, pure white blind cave crabs, and of course, the ubiquitous bats and swiftlets.

Lagan’s Cave

Mulu may be the most studied tropical karst area in the world, but the caves are certainly not giving up all their secrets. Despite over 40 years of exploration since the first Royal Geographical Society expedition in 1978 that revealed the world’s largest cave system by volume, we believe we have seen and mapped only about half of the subterranean passageways beneath the mountains of Mulu.

The best time to visit the park is in April and September, as not only are tourist numbers a little lower, the rainy season has not quite begun, so there is a much higher chance of safely reaching the caves in colourful longboats piloted by local Penan tribesmen. Seasonal temperatures in the jungle might not fluctuate all that much, but the caves are an even more stable environment. No matter how hot or cold it gets on the surface, it’s a constant 25°C with 99 percent humidity underground.

Entrance to Deer Cave; millions of bats exiting the cave at dusk in search of food

The objective today is to climb the inside of Easter Cave with expedition members Cookie and Tack. To get there, we need to head cross-country. Without the help of our local guides Veno, Jimmy and Wilson, we would be utterly lost. The jungle is a dangerous place for the uninitiated, but the skills of our guides, passed down from generation to generation from living within and around the park, will get us where we want to go. As we snake our way across the hillside, our guides explain the ways of the forest. A broken twig here, a carefully placed leaf there can speak volumes to those who know what they are looking for. Tiny pygmy squirrels dash up and down tree trunks, and discarded banana skins by our feet are all that remain of a monkey’s breakfast, while overhead, a flash of cream fur belies a snoozing giant cream squirrel.

Cave of the Winds

Despite the heat, we are all wearing long sleeves and gloves – a brush with the wrong sort of caterpillar could be quite painful! But we manage to avoid the creepy crawlies, and soon we arrive at the entrance of the cave. Swiftlets swoop and dive around us as Cookie, Tack and I turn on our lights and slip into the blackness for the two-hour commute to our current limit of exploration, edging around deep pits and shimmying down sandy climbs as bats wheel overhead.

Tack will be bolting this climb while I belay him. He kits up and plans his route as I place the first bolt to get things underway. It’s a long commute and the time we have underground is limited, so we need to be as efficient as possible. We drop into a routine: stretch, drill, hammer, clip, then up to the next one. Soon he’s at the top and rigging another rope so Cookie and I can join him up there with the kit needed to map the cave.

Professor Hazel Barton examining unusual stalactites in the entrance to Racer Cave

Part of our task as we explore these caves is to create accurate maps, both to show us how the system is developing and to allow others to return. Technology has come a long way from the 1970s and 1980s when teams were mapping the likes of Deer Cave, Lang’s Cave and Clearwater Cave that visitors to the park can now enjoy. Instead of tape measures and notepads, we now use laser pointers, and we sketch our rough maps on personal mini-computers as we go, ready for the full survey to be drawn up neatly on a desktop computer afterwards.

Back at the top of our climb, it’s time to start surveying. Before setting out from camp this morning, we thought we would intercept another cave called Sago, after the palm by its entrance – but the pristine floor tells us that no human has ever set foot here before. We pick our way up a muddy slope, past vivid crimson crystals and calcite formations, unlike any I’ve ever seen underground. One of the rewards of cave surveying is that you get to name a new passageway – we’ve called this corner of the world Pass Go.

A cricket in Stone Horse Cave

All too soon, our agreed return time looms, and we reluctantly pack away our kit for the journey back to the park headquarters. After so long in shades of near-monochrome, the colours of the forest are startling as we emerge. The leaves are greener than ever seemed possible, and the dusk chorus is so full of life that I can’t help but join in with a joyful whoop, elated at our day’s discoveries. We make our way back to the research centre to clean off, then it’s over to the park’s café for a hearty bowl of nasi goreng and to raise a glass of tuak rice wine to the day’s surveying – and to the caverns measureless awaiting us tomorrow.

The Pinnacles at Gunung Mulu National Park


Five fascinating facts about this jungle paradise:

  • Home to over 30,000 different species of plants, animals, birds, insects and fungi, the Gunung Mulu National Park is one of the most biologically diverse ecosystems on the planet.
  • The stunning stalactites you can see in Cave of the Winds and Lagang form at a rate of less than one millimetre per year – some of the structures within the caves are hundreds of thousands of years old.
  • Gua is the Malay word for cave, while gunung means mountain. Greet your Penan boatman in the morning with jian nivun and thank him by saying jian kenin. However, the Penan have no word for “please”, because in their culture, everything belongs to everyone equally.
  • Edward Shackleton – son of Antarctic explorer Ernest Shackleton – clearly inherited the exploration gene: he was the first man to summit Mount Mulu in 1932.
  • The rope bridge at the entrance to the park might look high, but after a thunderstorm, the river can rise by over four metres! This is a rainforest, after all.

Three more ways to spend time in the park:

  • Life in the Fast Lane

Guided tours to Lagang Cave are suitable for all ages and experience levels – but if you fancy something a little more adventurous, take a longboat upriver for the Clearwater Connection trip.

  • Live the High Life

Mulu offers some of Borneo’s best and most accessible jungle trekking. Check out the spectacular Pinnacles or get a bird’s-eye view with a canopy walk tour (dawn is the best time to go) – look out for rhinoceros hornbills, pygmy squirrels and gibbons!

  • After Dark

Don’t miss the nightly bat exodus at Deer Cave – each evening just before sundown, over three million bats stream out of the cave in tight formation and set off to hunt for their dinner.