The Hidden Valley is a stunning, cliff-lined closed valley on the east side of G.Api. Getting to this area is very arduous and, as a result, it is rarely visited. However it contains a number of very significant caves. Unlike the Melinau and Melinau Paku rivers, which flow through the limestone ridge in deep open gorges, the river that flows into the Hidden Valley sinks into the mountainside to go underground and it eventually comes to the surface again and joins the Melinau Paku River. Over the millennia, the waters that have cut their way through the caves on this side of the mountain have created both the largest underground chamber in the world and the deepest cave system in the National Park.

A unique feature of exploration in the Hidden Valley is the harshness of the surface terrain. Beyond the river sink, the ground rises up onto the limestone in a series of four huge depressions. Here the dense and often rotting vegetation covers heavily eroded limestone pinnacles, some rising to 30 m high. It can take several days to cover a few hundred metres and was described by the early explorers as “the most inhospitable terrain known to man”. Few who have visited the area in subsequent years would disagree!

The Hidden Valley has been explored by cavers only three times. Firstly as part of the 1978 RGS expedition and then, much later, by the two dedicated Hidden Valley expeditions of 1996 and 1998.

On the north side of the valley is the stunning, passage of Wonder Cave, full of huge formations. The cavers who explored it in 1978 reasoned that it was an ancient water course that has been bisected by the valley and predicted that its continuation would be found on the south side of the gorge. Sure enough, Prediction Cave was located - it is probably one of the largest segments of cave in the National Park; unfortunately it has been rammed full to the roof with river sediment. We know that the cave is so big because the wet sediment dried out and shrank away from the roof and walls, leaving a navigable cave some 100m wide but only a few metres high. In places it is possible to descend down the sides for about 40m before the sediment blocks the way on but it is impossible to determine the true depth of the passage. It is a very confusing cave to explore - easy to wander around in circles and very hard to find the way out.

A glance at the area cave map shows that Prediction Cave must have been connected to Good Luck Cave in the past. Unfortunately, a through passage has not yet been found and the entrance to Good Luck Cave is via the resurgence in the Melinau Paku Valley. The first half kilometre of the cave is occupied by a lake in a tall, narrow (5m wide) canyon in which the water levels vary from wading to totally out of depth. Beyond this is a beautiful, turbulent streamway which includes a 15m diameter whirlpool which can be difficult to cross against the current. From this point on, the roof disappears from sight and the streamway narrows, forcing explorers to traverse along the walls until it widens out into a steep slope of boulders. From here it is up, up, up, and then up, up, up some more! All the while the roof soars out of sight and the walls radiate out into the distance and beyond. It is impossible to tell when one actually enters the 700 x 400m Sarawak Chamber and its size can only really be gauged by spreading out a team around the treacherous, boulder-strewn floor and listening to whooping echoes travelling on into the distance; this is, after all, the largest underground chamber in the world! On the far side of this void a small streamway continues to a sump, the water originating from the Hidden Valley River.

Back in the Hidden Valley, if you pass the river’s flood sinks and go up onto the limestone, you enter the series of large depressions. High up on the southern side of the second depression are located the highest entrances to the Bridge/Cloud/Cobra System This is the deepest cave in the National Park and has a vertical range of 460m and a length of 15.5km. It’s easy to speculate that these upper passages (the ‘Tombraider series’) form the oldest known outlets for the Hidden Valley waters. They are a complex mix of chambers, climbs and short sections of passage, never more than 10 x 10m in diameter. The two entrances at the highest part of the cave were only discovered from the inside and have never been located on the surface!

The route to the main level of Bridge Cave passes through some small, 1 x 1m passage and a tight squeeze. One of the small passages leads back north to emerge only a few tens of metres above the base of the second depression, close to Arch Cave which was probably once associated with the Bridge System. Much of the cave is typified by white, deeply scalloped walls and roofs with floors covered in deep guano deposits from the large swift populations that live here.

Bridge Cave is connected in two places to Cloud Cave down deep pitches. One of them, a huge 30m diameter, 95m deep shaft, has never been descended despite two attempts, although markers dropped down it were found at the base of a huge aven in Cloud Cave, proving the connection.

The entrance to Cloud Cave lies near the base of the fourth depression and is so large that clouds form in the entrance. The main passage is huge, being 80m high and 45m wide and heads back towards the Hidden Valley to end in a total choke after a kilometre. Interestingly this choke is 100m below the level of the present alluvial sinks of the Hidden Valley water, leading us to wonder if this was the original conduit of the Hidden Valley water or does it predate the valley and continue under the summit of Gunung Api?

An obscure duck under a hole in the wall of a small stream passage provides a connection through to Cobra which was explored from the camp in the Melinau Paku valley in 1984. The explorers turned back at the end of Cobra, thinking that the passages were closing down ­ a few more metres and they would have found their way into the Cloud System! From the Cloud connection the trip to the Cobra Entrance is about 3km in passage mostly 10 to 15m in size, including a swim through a low arch which has a nasty habit of sumping after rain - at least one group of explorers has had a long wait here! Beyond this is another section of streamway before a climb up pitches leads to the easier romp to the entrance along a passage with an undulating floor of sediment banks and powdery guano, interspersed with clean boulders and fangs of rock. Near the entrance is the ‘Cobra’ himself, a tall stalagmite resembling a snake’s head, complete with fangs.