Just up the Melinau from Cave of the Winds, the 1978 caving team members noticed a tributary of translucent green water mixing with the light brown of the main river flow. Although there was no record of a cave here, the tributary was followed for a short distance and the water was found welling out of the mountain, through large impenetrable blocks. Knowing that a cave must lie behind this boulder collapse, the cavers began to explore the hillside and 30 metres above lay a wide open entrance. And so began the exploration of Clearwater - which over the years was to become one of the world’s top ten longest caves.

From the entrance, a boulder slope leads down to the river and to the left is the resurgence chamber which is fed by two rivers. The smaller of the two, Goldwater, picks up drainage from around the edge of the mountain to the north and is lightly coloured by tannins from fallen leaves. The bigger flow, from deep inside the mountain, is Clearwater, one of the biggest known underground rivers - in flood, 150,000 tonnes of water pour through the passage every hour. From this junction, the river passage continues for well over a kilometre, with impressive dimensions averaging up to 30 metres in diameter. For the explorers, this was very exciting caving as progression entailed crossing and re-crossing the river a number of times, avoiding razor-sharp blades of rock beneath the surface. Eventually the magnificent passage ends at River Junction, where it sumps. From here there is a network of passages bypassing the flooded area and leading to a massive complex of dry passages above. The biggest of these, Revival, bores its way north as a massive phreatic tube, with an average passage height of 45 metres and a width of 30 metres.

At the end of the 1977-78 expedition the team concluded that, 'with its many open passages and pits still to be explored, the final length of Clearwater could be prodigious. Throughout the cave, the passages are enormous by world standards'.

In spite of Cave of the Wind’s close proximity to Clearwater (less than half a kilometre), it wasn't until the 1988 expedition that the connection, entailing an obscure, tight squeeze between boulders, was made between Illusion, in Cave of the Winds, and King Seth's Maze in Clearwater. The combined length of the two caves instantly thrust the Clearwater System up the ranking of the world’s longest caves. Nowadays, the trip from Wind to Clearwater is one of the classic adventure caving trips on offer in Mulu ­ negotiating some of the biggest passages in the world, weaving between huge stalagmite columns and exiting down the stunning, exhilarating Clearwater River passage.

Over the years, continuing exploration of Clearwater has revealed a further 6 sections of the river, separated by sumps. The upstream limit of exploration, Clearwater 7, is extremely remote and the bypass to the last sump is highly flood-prone and can be closed off by high water for months on end. The active river sections are all linked into the multi-layered, dry cave passages above, forming a complex maze which spans a vertical range of 350 metres. In many of the high level relict passages there are huge banks of infill, providing evidence of previous phases in the cave's life when sediment has been forced into the cave under pressure from floods and landslides on the surface. Now these sediments block or impede exploration.

To the north of Revival, the Dune Series and Troll Chamber eventually link to the southern end of Blackrock and, close to the junction of the two systems, a tiny connection provides the ‘North Entrance’, a tight squeeze out to the surface which has only once been located from the outside, the entrance being an inconsequential rift between rocks, hidden in the forested hillside.

Midway along Revival, an ascent up a massive boulder slope leads to the Scumring, a pool of water or mud (depending on recent rainfall) from which two huge pitches are believed to lead down to the river far below; the second pitch has never yet been descended. The flat area around Scumring provided an excellent underground camp site and from here the 15km Armistice Series was explored ­ another network of passages leading north at high-level to an end point only 800 metres from Whiterock's Api Chamber. The floors of many of the passages here are littered with the bones of dead bats ­ maybe an indication that a collapse sealed off an old entrance, entombing the animals in the cave, or perhaps just the accumulation over thousands of years of individuals that were lost and died of thirst or starvation. At an even higher level, The Secret Garden, RMAF Hole and Solo are massive surface features on the mountain slopes and, of these, only the Secret Garden has a fully explored link to the passages below.

As well as the entrance at the resurgence and the tiny Northern Entrance at Troll Chamber, access into the system is possible at a number of other points along the foot of G.Api on the edge of the Melinau’s alluvial plain. The best known of these is Snake Track - its small beginnings exploding into the well-decorated grandeur of the north end of Revival. This entrance takes its name from polished tracks across the cave floor, highways for snakes, porcupines and rats entering and leaving the cave. Access into Clearwater 3 is also possible from the Melinau Paku Valley and, close by, a voice connection has been made between Drunken Forest Cave and Alexander Palace, a small, well-decorated chamber that was found during the exploration of Armistice and the river passage of Clearwater 5.

Clearwater is the typical Mulu cave system on the grandest of scales - high energy active river levels and multi-layered relic cave above. This complex system has continued to reveal its secrets over 30 years of exploration and now, with Blackrock and Whiterock, comprises 151.4km of surveyed passage. Despite 3 decades of effort, there is little doubt that a lot more of this cave remains to be discovered.