What it is
What Maliau Basin actually is
Maliau Basin is a roughly circular bowl of primary rainforest in south-central Sabah, in the Tongod District, about 40 kilometres north of the Kalimantan border and 190 kilometres inland from Tawau. It is often described as a crater, which is understandable and wrong. There was no impact and no volcano. The basin is a sedimentary formation, built from gently inclined beds of sandstone and mudstone, and the resemblance to a caldera is a coincidence of erosion.
What makes it unusual is the wall. The perimeter is delimited on almost every side by cliffs or extremely steep slopes, which makes the basin effectively unclimbable on foot from most directions. That single geographic fact explains everything else about the place: no one ever settled inside it, no logging concession ever worked it, and it stayed off the map long after the rest of Sabah had been surveyed. The first proper scientific expedition into the interior only took place in 1988.
The whole thing is a single water catchment. Every stream inside the rim feeds the Maliau River, which escapes through a gorge in the south-east corner into the Kuamut, and eventually into the Kinabatangan, Sabah's longest river. The water runs a dark tea colour, stained by tannins leaching out of the peaty leaf litter and heath forest above.
How big Maliau Basin is, exactly
The numbers get quoted loosely, so it is worth separating them. The Maliau Basin Conservation Area covers 588.4 square kilometres, or 58,840 hectares. Of that, the basin itself accounts for about 390 square kilometres. The remaining 198.4 square kilometres is forested land to the east and north of the rim, added when the boundary was extended, and it includes Lake Linumunsut, thought to be Sabah's only freshwater lake that is not an oxbow.
The highest point sits on the northern rim at over 1,675 metres above sea level, and by the admission of the conservation area's own management it has still never been accurately surveyed. That tells you most of what you need to know about how thoroughly this place has been documented.
Why it is called the Lost World
The nickname is a tourism label, but it earned it. Roughly a third of the basin is open to visitors. Less than half has been properly explored by researchers. Around 70 kilometres of marked trails exist, which sounds like a lot until you set it against the area they cover.
The Murut name for the area translates roughly as the land of the giant staircase, a reference to the stepped landscape and the extraordinary number of waterfalls stacked down it. Maliau has more waterfalls than anywhere else in Malaysia.
Maliau and Danum Valley were protected in the same year, 1981, under Yayasan Sabah's long-term forest management plan. Danum became a world-famous research station with a lodge, an airstrip route and a canopy walkway. Maliau did not. In 1997 the Sabah State Legislative Assembly gazetted it as a Class I Protection Forest Reserve, the highest level of protection available, and it has stayed deliberately hard to reach ever since.