From a treetop-level bird "hide" - for discreet observation of avian life - I gaze upon a stunningly beautiful jungle backdrop.
A big-beaked hornbill, a member of one of the park's eight hornbill species, sits on a nearby tree branch. Sarawak terms itself "land of the hornbills", with these strange-looking creatures plentiful in the park.
I wander along a 480m jungle canopy walkway, which park officials call earth's longest tree-based crossing. The rainforest and its rivers support an array of wildlife: bearded pigs, pangolins, crocodiles, squirrels, sun bears, barking deer (and other antelope types).
Next day, I sample a chunk of Headhunter's Trail, one of many hiking trails and named for a long-gone custom of beheading enemies.
In the evening, a night cruise up the Melinau River has guides identifying insects, frogs and nocturnal birds.
But what attracts most visitors is a collection of caves. This 52,000ha reserve is pockmarked by spectacular cavities - including Sarawak Chamber, the world's largest cave, which boasts an interior so cavernous that 40 jumbo jets could park inside.
But here's the catch. The Sarawak Chamber is one of several hard-to-get-to "wild caves" without lighting or pathways.
Park authorities admit only those proving they're experienced cavers (sometimes with letters from foreign caving clubs) and agreeing to be accompanied by guides. Gruelling hikes often precede visits to "wild caves" with narrow crevices and tunnels.
The Pinnacles, a natural rock formation in Borneo's Gunung Mulu National Park. Photo / Getty Images
Four so-called "show caves" are an altogether tamer - and more popular - diversion. Exploring is eased by boardwalks and lighting.
That's why I find myself on one boardwalk, traipsing through the Borneo jungle to Deer Cave, by far the most visited, where I transfer to another and venture inside.
Lighting is cunningly subdued. Birds called swiftlets dart hither and thither.
A non-venomous racer snake, brilliantly blue-hued and half a metre long, crosses the boardwalk ahead of me. It's one of many living in the cave, surviving on a diet of bats. And the bats are plentiful.
The tiny mammals' high-pitched squeaks are the dominant sound. Three million bats exit the cave at dusk, past a boardwalk crowded with humans, to gorge on insects.
A giant cloud, comprising tens of thousands of bats, passes above. Guides say this daily exodus is a key reason people come.
Far smaller than Deer Cave is Lang's Cave where cleverly positioned lighting accentuates stalactites and stalagmites resembling abstract sculpture.
The two remaining "show caves" definitely merit visits: Clearwater Cave - Asia's longest, which is partly navigable by boat and favoured by tourists for subterranean swims - and Wind Cave, where breezes waft through a vast chamber called the King's Room, which has artistically illuminated limestone formations.
Gunung Mulu, shrouded by Borneo's jungle, is easily reached from Malaysia's capital, Kuala Lumpur.
A Swedish tourist tells me over breakfast in the park's resort: "It's wonderful that so many people mistakenly put this place in the too-hard basket - it means it's less crowded."
I can't help but agree.
WHERE TO SEE THE RENOWNED ORANGUTANS
Despite being one of Asia's best jungle destinations, Mulu isn't famous for viewing Borneo's renowned orangutans or proboscis monkeys in jungle settings. But itineraries are easily bent to include Kuching, Sarawak's capital, between Kuala Lumpur and Miri. From Kuching, half-day tours visit habitats of both. See sarawaktourism.com for more details.
CHECKLIST
Getting there: Malaysia Airlines flies six times a week from Auckland to Kuala Lumpur, with return economy tickets available for from $1383 in their "Spring Sensation" deal, on sale until September 21.
Accommodation: Royal Mulu Resort, with rooms overlooking the Melinau rover, is a 45-minute boardwalk stroll to Deer Cave. It will be rebranded as Mulu Marriott Resort and Spa later this year. Properties just outside the park have budget accommodation.
The writer was a guest of Sarawak Tourism.
