From Brunei we took a short 6 hour bus ride back into Malaysian Borneo, crossing the border at Kuala Belait. I then spent 16 hours holed up in a hostel in Miri with the tail-end of a bout of food-poisoning, willing myself well in time for a tour to the Niah Caves that we had booked for the following morning. Sarah tells me she spent a lovely afternoon exploring the 'sights' (browsing the shops and getting a pedicure).

Aside from a mild fever virtually indistinguishable from my base-level of tropical sweatiness, the next morning I was good to go and we set off with our guide Ken west on the coastal road to the limestone cliffs of Niah.

The Niah Caves are the result of similar limestone rock formations to those we saw in Halong Bay in Vietnam – they're enormous humps of rock, big sore thumbs in an otherwise flat jungle landscape. In Halong Bay water still laps the base of these rocks, in Niah the sea has rolled back several miles leaving jungle and more palm oil plantations between the beach and the caves.

Niah and its big brother Mulu (another set of caves in Sarawak) housed some of the oldest recorded human settlements in the region. Inside the mouth of the Great Cave at Niah, 'modern' human skulls have been found in a highly organised burial ground that dates back 40,000 years – the oldest in the whole of S.E.Asia – as well as a series of very faded cave paintings of boats and monsters dated at 20,000 years old.

We strolled the 4km from the camp to the caves, protected from the rainforest by a raised wooden walkway, before climbing the wooden staircase up the face of the limestone cliff to the enormous cave entrance. Once inside the mouth of the Great Cave, sunlight quickly became a distant memory and our torch did little to penetrate the heavy darkness around us. Following our guide's heels, we felt our way over damp wooden walkways deeper into the cave system. Only the clicks and screeches of bats and the echoes of dripping water gave us an idea of the scale of the space, until we rounded a corner and saw light lancing down from holes in the cave roof 50 metres above us.

Tourism in the Niah caves exists side by side with two older industries – guano (bat shit) farming and birds' nest collecting. Nitrate-rich guano from the 2-3 million bats living in the caves is shovelled up daily by groups of locals living in camps within the caves and sold as an expensive natural fertiliser. The ammonial stench hit us like waves in the more popular bat-caves. I've no idea how the farmers live with it day-in day-out.

Bird's nest collecting is a less disgusting but much higher-risk profession. Collectors ascend 100 foot high bamboo poles to knock down Swiftlet nests attached to the cave roof. The saliva that the birds use to construct their nests has supposed aphrodesiac properties that make them the most valuable ingredient in Chinese 'bird's nest soup'. It is this rich export market that causes otherwise sane locals to climb to such heights with no ropes.

Both these professions require their practioners to live in the caves in semi-permanent camps to guard their territories from interlopers. As we walked through the caves, we came across a guano farmer standing by the walkway on a rock in the milky half-light, smoking a cigarette and looking very much like an overgrown Golem – wearing only shorts and a non-functional head-lamp, he was skinny and pale with clammy-looking skin that stretched over a little pot belly. After our guide exchanged a few words with him ('Nice weather'?!?) we walked quickly on.