Academic Reading — Test 192

Reading passage

A. In the interior of Sarawak, on the island of Borneo, a range of forested limestone hills conceals one of the most extensive networks of caves yet documented on Earth. Gunung Mulu National Park, named after the mountain that dominates its skyline, was declared a UNESCO World Heritage Site in 2000 in recognition of the exceptional scale and diversity of its underground landscape. The park sits within dense equatorial rainforest and remains accessible chiefly by light aircraft or by boat along winding rivers, a remoteness that has kept its caves comparatively undisturbed by human activity until recent decades. Beneath the forest canopy lies a labyrinth of passages, chambers and shafts carved into rock that began forming as marine sediment many millions of years ago, long before the mountains above it were lifted into their present shape. B. The caves of Mulu owe their existence to limestone, a rock composed largely of calcium carbonate that is slowly but persistently dissolved by rainwater. As rain falls through the atmosphere and filters through decaying vegetation on the forest floor, it absorbs carbon dioxide and becomes mildly acidic. This weak acid seeps into cracks and joints within the limestone, gradually widening them into fissures, then tunnels, and eventually into chambers large enough to swallow a cathedral. Over an immense span of geological time, the process has produced not only horizontal passages but also towering vertical shafts where water has plunged downward in search of the lowest available outlet. The result is a three-dimensional maze whose true extent has still not been fully mapped, despite decades of survey work by speleologists from many countries. C. Among the park's most celebrated features is Deer Cave, whose main passage is regarded as one of the largest cave passages in the world by volume, a chamber so vast that it could enclose several large aircraft hangars. The cave takes its name from the barking deer that once sheltered within its entrance, drawn by mineral-rich mud deposits inside. Today, however, Deer Cave is better known for a different resident: an estimated three million wrinkle-lipped bats roost on its ceiling, clustered so densely that their combined weight has, over time, stained the rock dark with droppings. Each evening, as daylight fades, these bats stream out of the cave mouth in a long, spiralling column, a phenomenon visitors travel from around the world to witness. Researchers believe that flying in this dense, swirling formation makes it harder for predatory bat hawks circling outside to single out and seize an individual bat from the crowd. D. The droppings left behind by the bats, together with those of the cave swiftlets that nest on the ceilings of several Mulu caves, accumulate on the cave floor as thick deposits of guano. Far from being a lifeless by-product, this guano sustains a surprisingly rich community of cave-adapted invertebrates, including specialised cockroaches, earwigs and centipedes that feed directly on the droppings or on the fungi growing upon them. These creatures, in turn, support small predators adapted to a life spent permanently in darkness, with reduced eyesight and heightened reliance on touch and smell. The guano ecosystem illustrates a fundamental ecological principle: that even in an environment with no sunlight and therefore no plant growth, an entire food web can flourish, provided that energy is imported from outside, in this case carried in on the wings of bats and swiftlets that forage in the forest beyond the cave. E. Elsewhere in the park lies the Clearwater Cave system, named for the underground river that flows through much of its length, and recognised as one of the longest connected cave systems anywhere in the world. Cavers continue to find new passages linking it to neighbouring caves, extending its mapped length year on year. Deep within a separate cave known as Gua Nasib Bagus is the Sarawak Chamber, widely cited as the largest known cave chamber on the planet in terms of floor area, large enough that its full dimensions are difficult to appreciate even when standing inside it with a torch. On the flanks of nearby Gunung Api, a different but related process has produced the Pinnacles, a cluster of razor-edged limestone spikes that rise abruptly from the forest slope. These formed as rainwater dissolved the rock unevenly along natural joints and fractures, leaving the harder, more resistant ridges standing proud while the surrounding stone was worn away. F. For the indigenous Penan and Berawan communities who have long lived around the fringes of Mulu, the caves were never simply geological curiosities; they were sources of practical livelihood and, in some cases, of spiritual significance. Swiftlet nests, built largely from the birds' hardened saliva, were traditionally harvested from cave ceilings for trade, a practice carried out with considerable skill and risk given the heights involved. Today, conservation regulations restrict such harvesting in much of the park, reflecting a shift toward protecting the caves' ecosystems for scientific study and ecotourism rather than extraction. As researchers continue to push into unexplored passages, Gunung Mulu remains a reminder that, even in an age of satellite mapping, substantial parts of the planet's interior are still being discovered for the first time.