Hearing the soft calls, woo woo-ooh, I look up, searching the canopy for gibbons. Above the soaring tree trunks, there seems to be only dense foliage. Our guide points up and there's movement: a gibbon swings from one branch to another, halts and casually hangs from one hand while picking small fruit with the other. It's a handsome animal, silvery grey with a black mask, and a supreme natural gymnast. Though gibbons are found in much of Southeast Asia, this is an endangered species, the Javan gibbon- and we're in its prime remaining stronghold, Gunung Halimun National Park. The park is named after the Gunung Halimun massif, where hills of extinct volcanoes help safeguard forest containing a wealth of rare plants and animals. Despite being just 100 kilometres south of the Indonesian capital, Jakarta, it took almost a day to travel here by road. Gunung Halimun attracts far fewer visitors than the nearby Gede and Pangrango volcanoes. We're staying in a village tucked into a valley and nestled between tea plantations and forest. Though it's a base for eco-tourism, only 2,000 people visited last year. Here, there are no signs that Java is the world's most populous island. Forest trails pass huge trees, ferns with enormous fronds suspended on two-storey-high trunks and streams rushing over cascades and waterfalls. Wildlife can be elusive but there are flocks of birds, including turquoise flycatchers, and an eagle soaring overhead. Even when not in view, gibbons announce their presence. Males and females burst into duets, with wavering, whooping cries that carry across valleys as if in exuberant celebration of the forest. Another national park is close by. On the map, that is. We travel to it via Jakarta, taking a highway towards westernmost Java. It's an easy journey to the coast, there's a decent road to the south. On the outskirts of the park, however, we can only lurch slowly along a stony track where heavy rains have turned potholes into puddles and ponds. This park, Ujung Kulon, is a wild, remote place, partly because of poor transporta- tion. Malaria is another factor. But also because the park includes remnants of Krakatoa, a volcano that exploded in 1883, unleashing devastating tsunamis. Afterwards, few people resettled and the land reverted to jungle. Ujung Kulon was a final refuge for the Javan tiger, last seen in 1972. Today, it is best known as the main haunt of the world's most endangered large mammal, the Javan rhino. Extirpated from most of Southeast Asia, poached for its horn and other products used in Chinese medicine, only about 50 are left roaming the forest here. The rhinos are shy and rarely seen. Our guide, Pak Komar, says he has seen them only a handful of times; his father was a park ranger and was once chased up a tree by a female rhino, spending the night there while she circled menacingly below. Komar takes us on a jungle river ride. Our narrow wooden canoe seems to be on the verge of toppling over. We paddle past mangrove trees, watching for wildlife, yet the forest seems still. Komar informs us that crocodiles are a common sight here and, in the dry season, pythons hang from branches over the water. We come to a bend in the river. 'The rhinos like to take a bath here,' says Komar. On the muddy riverbank, he spots a rough print that suggests a rhino bathed recently. From the canoe, we clamber on board a slender wooden boat that will take us to an island in western Ujung Kulon. It bounces across the choppy sea before turning into calmer waters and mooring at a pier in a jungle-ringed lagoon. Here, there's a visitor centre, with wooden lodges set around a grassy clearing. Lunch is served, and monkeys jump onto chairs outside the window, intently watching our every move. They look menacing but return to the shade of the trees when food is no longer in view. Deer emerge from the trees to graze on the lush grass, showing little concern about the humans present. They're joined by wild boar and peafowl with their extravagant tail plumes. A two-metre-long monitor lizard strolls up from the beach and saunters across the glade. Later we walk beneath strangler figs supported by tangles of thickened roots, arriving at a limestone headland perched above crystal clear water. Precipitous cliffs mark a fault line, where coral was thrust from the sea in one of the violent events that periodically occur in this geologically active region - as well as Krakatoa, we're near the fault that caused the 2004 Boxing Day tsunami. A boat ride and a short hike away, is a lighthouse, atop the very western tip of Java, where mighty waves roll in from the Indian Ocean and pound volcanic rock. The keeper here might well have the loneliest job on Java. He invites us into his quarters, glad of the few moments of company, and explains that his colleagues are unwilling to stay here out of fear of malaria - and ghosts. Near the lighthouse, piles of dung show cattle have passed nearby. Not the domestic sort, though - these are wild banteng. In the late afternoon, Komar guides us along a short trail to a clearing ringed by forest. On the other side, the cattle can be clearly seen. They all have white 'stockings'; so are much more fashionable than the feral cattle that roam the Hong Kong countryside. I crouch behind my tripod, moving slowly in case the banteng should suddenly dash for the trees- or even charge at me. But they graze peacefully and, with evening approaching, a fruit bat the size of a buzzard flaps overhead. It's time to leave. Next morning, we head back to the mainland and away from this lost world at the tip of Java. Getting there: Cathay Pacific ( www.cathaypacific.com ) flies from Hong Kong to Jakarta, from where the trip to Gunung Halimun can be arranged through Teguh Hartono (e-mail: firstname.lastname@example.org ).