Gunung Mulu National Park: A Cave World

Gunung Mulu National Park contains some of the most remarkable karst geology on Earth. Hidden deep in the rainforests of Sarawak, Borneo, this UNESCO World Heritage Site is famous for its phenomenal karst features. It is home to the world’s largest natural cave chamber (Sarawak Chamber), one of the largest cave passages (Deer Cave), and the razor-sharp limestone spikes known as The Pinnacles. Access to the park is primarily by air, adding to the sense of isolation and adventure. Above ground, the primary rainforest is ancient and teeming with life; below ground, a labyrinth of over 295 kilometers of explored caves hides secrets millions of years in the making.

The Show Caves

Mulu has four “show caves” accessible via plank walks and guided tours, showcasing the diversity of the underground world.

• Deer Cave: Immense is the only word to describe it. The main chamber is over 2 kilometers long and 174 meters high—large enough to fit St. Paul’s Cathedral five times over. It was once a shelter for deer (hence the name), but today its most famous residents are the bats.
• Lang Cave: Smaller and more intimate, located right next to Deer Cave. It features stunning stalactites, stalagmites, and “shawls” that look like frozen stone curtains.
• Wind Cave: Named for the cool breeze that blows through its narrow constrictions. It features the “King’s Chamber,” a spectacular display of pillars and columns.
• Clearwater Cave: Part of one of the longest cave systems in Southeast Asia. An underground river roars through it. Visitors can swim in the crystal-clear water at the picnic deck outside the cave entrance.

The Bat Exodus

One of Mulu’s most spectacular natural events occurs at dusk. Millions of wrinkled-lipped bats stream out of Deer Cave in a spiraling, ribbon-like formation to hunt for insects in the rainforest. This “Bat Exodus” can last for nearly an hour. Visitors gather at a purpose-built observatory to watch the show, while Bat Hawks swoop down to snatch a dinner on the wing. It is a striking display of nature’s abundance.

The Pinnacles

For hikers, the ultimate challenge in Mulu is the trek to The Pinnacles. These razor-sharp limestone spikes rise up to 45 meters high through the forest canopy on the slopes of Mount Api.

• The Trek: It is a grueling 3-day/2-night adventure. It involves a boat ride, a hike to Camp 5, and then a steep, near-vertical climb using ropes and ladders to reach the viewpoint.
• The Reward: The view of the grey stone spikes jutting out of the green jungle is found nowhere else. It looks like a stone forest planted by giants.

The Headhunters’ Trail

For those who want to follow in the footsteps of history, the Headhunters’ Trail combines trekking and longboat travel. It follows an old warring path used by Kayan headhunting parties to launch raids on the people of the Limbang area. Today, it is a peaceful way to exit the park overland, staying in remote Iban longhouses along the way.

Flora and Fauna

Mulu’s rainforest is incredibly diverse.

• Insects: Mulu is famous for its weird and wonderful bugs, including the Lantern Bug with its long, colorful snout and the Raja Brooke’s Birdwing butterfly.
• Plants: The park has 17 vegetation zones. You can see rare orchids, carnivorous pitcher plants, and the massive Tapang trees, which host giant honey bee combs.

Practical Information

• Getting There: The only practical way to reach Mulu is by flight (MASwings) from Miri, Kuching, or Kota Kinabalu. The flight offers stunning views of the winding rivers and jungle canopy.
• Guides: You must have a licensed park guide for all cave tours and treks (except the botanical loop). Booking in advance is highly recommended.
• Accommodation: Options range from the luxurious Mulu Marriott Resort (just outside the park) to the park’s own comfortable bungalows, longhouse rooms, and hostels.
• Connectivity: Wi-Fi is available at the park HQ and the Marriott, but it can be slow. There is no cell signal in most of the park.

Frequently Asked Questions (FAQ)

Are there leeches?

Yes, especially on the treks to Camp 5 or the Pinnacles. Leech socks are your best friend.

Is the cave water safe to swim in?

The water at the Clearwater Cave picnic deck is safe and refreshing. Do not swim inside the caves unless on a specific adventure caving tour.

Can anyone see the Pinnacles?

No. The climb is physically demanding and technically difficult. Rangers can turn you back if you don’t reach the checkpoints in time. You must be fit.

Do I need a headlamp?

Yes! Even for the show caves, a torch is useful. For adventure caving, a headlamp is mandatory.

What is the “Sarawak Chamber”?

It is the largest known enclosed space on Earth. It is not a show cave; visiting it requires an “Adventure Caving” expedition for experienced cavers only.

Adventure Caving: Beyond the Show Caves

For visitors with experience and a spirit of exploration, Gunung Mulu offers several “adventure caving” options that go far beyond the illuminated boardwalks of the show caves. These must be booked in advance through the park, and a trained guide is mandatory.

Lagang Cave: One of the most accessible adventure options. The route involves wading through shallow water and squeezing through narrow passages to reach vast chambers decorated with cave coral and rare cave pearls. It is suitable for reasonably fit adults with no previous caving experience.

Racer Cave: Named for the cave racers—slender, fast-moving snakes that hunt cave swiftlets and their eggs deep in the darkness. The cave is home to a large swiftlet colony, and the route passes through chambers streaked with guano and alive with the chirping of thousands of birds. It is a visceral, slightly chaotic experience that captures the full ecological drama of Mulu’s underground world.

Clearwater Connection: For experienced and fit cavers, this demanding route connects Clearwater Cave to Cave of the Winds through underground passages, some of which require swimming through flooded chambers. It is one of the most adventurous caving experiences available to non-specialist visitors anywhere in the world.

Sarawak Chamber Expedition: At the extreme end, an expedition to the Sarawak Chamber—the largest known cave chamber by area on Earth—takes two days and requires a physically demanding approach through challenging terrain. Participants must have solid caving experience and are required to pass a fitness assessment. The chamber’s scale is so overwhelming that it cannot be comprehended from a single vantage point; even with powerful torches, the far walls vanish into darkness.

The Geology of Mulu: Limestone and Time

Gunung Mulu’s cave systems were carved by slightly acidic rainwater dissolving the thick beds of Melinau Limestone that dominate the park. This process, known as speleogenesis, began roughly two million years ago and is ongoing. The limestone was originally deposited on the floor of a shallow tropical sea; the fossils of ancient marine organisms—crinoids, corals, and bivalves—can be seen in the rock walls of many of the cave passages.

The caves have been shaped by two main processes. In the phreatic phase, water filled the cave passages completely, dissolving the limestone in smooth, rounded tubes. In the vadose phase (which is where most of the caves now exist), the water table dropped and the passages began to drain, with a stream cutting downward through the floor of the old phreatic tube. The result is the characteristic “keyhole” cross-section seen in passages like Deer Cave: a round ceiling (from the phreatic phase) sitting above a narrow slot at the bottom (cut by the vadose stream).

The surface features—The Pinnacles and the jagged, knife-sharp “razor karst”—are formed by a different process: rainwater dissolving the limestone surface layer along micro-fractures, with the harder rock between the fractures remaining as towering spikes. The flora that survives in this environment is remarkable, including several species of pitcher plant adapted to extract nutrients from decomposing insects in an environment where the soil is almost non-existent.

Wildlife of the Forest: Canopy and Ground

Mulu’s 295 kilometers of explored cave passage are extraordinary, but the rainforest above ground is equally rich. The park lies within the Sundaland biodiversity hotspot and contains an exceptional diversity of species across all taxonomic groups.

Primates: Bearded pigs, long-tailed macaques, and silvered leaf monkeys are commonly seen near the park headquarters. On the trail to Camp 5, proboscis monkeys—the strange, big-nosed primates endemic to Borneo—are sometimes spotted crossing the river at dawn. Gibbons call from the canopy in the early morning with a haunting, whooping song that carries for kilometers.

Birds: Over 270 species have been recorded, including eight species of hornbill. The rhinoceros hornbill, with its enormous curved casque, is the most spectacular and is frequently heard crashing through the canopy before being spotted. Broadbills, pittas, and trogons reward patient early-morning birders along the forest trails.

Night Life: Night walks along the plank walks near the park headquarters reveal a hidden world of huntsman spiders, stick insects, sleeping lizards, and the occasional slow loris—a primate with huge reflective eyes, adapted for nocturnal life. A flashlight and a quiet step reveal more biodiversity in an hour of night walking than a full day of daytime hiking.

Planning Your Visit: A Suggested Itinerary

Mulu rewards a minimum of three nights, and ideally four to five if you plan to do The Pinnacles trek.

Day 1: Fly in from Miri (30-minute flight). Check in and rest. Afternoon: Show caves tour (Deer Cave and Lang Cave). Evening: Bat Exodus at the observatory.

Day 2: Morning: Clearwater Cave and Wind Cave boat trip. Afternoon: Free time for swimming at the Clearwater picnic area, or a short nature walk on the botanical loop.

Day 3–5 (Pinnacles option): Boat to Melinau Gorge, hike to Camp 5, summit attempt to The Pinnacles viewpoint, return to park headquarters.

Alternative Day 3: Headhunters’ Trail (for those exiting to Limbang rather than returning to Miri).