A visitor taking the plank-walk out from Park HQ passes easily through an area of swamp-forest and gradually becomes aware of a huge limestone wall ahead and to the left, a wall in which small cave entrances can be seen with old stalactites hanging down. Further on, another wall becomes visible to the right and the walker becomes conscious of being channelled into a huge corridor.

However, this is no simple valley. After passing a wooden structure and a small amphitheatre, where visitors can sit and rest, the walls come together above a huge rock arch. This is the entrance to Deer Cave, the largest single cave passage yet discovered on earth. Its scale is truly awesome - an immense pile of boulders obscures the true width of the entrance, over 170m, whilst the arch of the roof rises to some 120m; nowhere in the cave is the roof less than 90m high!

The cave is home to several million bats and their droppings form huge piles of guano, a food source for multitudes of cockroaches, beetles and other insects which scurry across the surface of the dry brown powder and consume dead or dying bats that have fallen to the floor. These insects, in turn, provide the food for other invertebrates ­ centipedes and spiders, some as big as a hand, others hidden deep in white tunnels of web, This is a complex network of life, dependent on a daily spectacle when a stream of bats pours out of the cave, swirling across the sky and visible for kilometres. From the surrounding cliffs, hawks and peregrines dive into the stream to catch their meals and consume them in flight.

A concrete path takes the visitor deep into the cave, avoiding the worst of the guano and the streams of water that cascade down in wet weather from the roof, high, high above. On the right wall a steep ramp leads up to a higher level, whilst the main passage takes a massive dog-leg and the path leads up to a wooden viewing platform. From here, the noise of the river can be heard, well below, as it winds its way between rocks before entering a small tunnel from which it resurges into daylight, near the entrance. However, today’s river is a mere trickle compared to the enormous volumes of water than once carved this immense cave through the mountain.

From the viewing platform a stunning view opens up across a boulder-strewn floor and beyond to the eastern entrance of the cave. The dark colours of the rock are contrasted by the brilliance of the forest, visible out in the ‘Garden of Eden’, the closed valley which was probably once a huge underground chamber that collapsed thousands of years ago. On the other side of this valley, a dark scar on the distant cliff marks the entrance to Green Cave, the upstream continuation of Deer’s ancient river passage, which cuts through the massif and eventually opens into the side of the Melinau Paku River valley.