Mulu’s Sarawak Chamber

The Sarawak Chamber is one of the largest underground chambers on Earth: 600 metres long, 415 metres wide, at least 80 metres high, a volume of around 12 million cubic metres, and an unsupported roof span of 300 metres. That last figure is the one that stops engineers in their tracks. Three hundred metres of rock holding itself up over nothing.

It sits in Gunung Mulu National Park in Sarawak, Malaysian Borneo, and it is not even the park's only superlative. Deer Cave is the largest cave passage in the world, between 120 and 150 metres in diameter. Clearwater Cave is the longest cave system in Southeast Asia, with 227.2 km of passage explored as of October 2018. Gua Nasib Bagus, Benarat Cavern and the Cave of the Winds are all here too.

The park is dominated by three mountains: Mount Mulu at 2,376 m, which is sandstone, and Mount Api at 1,750 m and Mount Benarat at 1,858 m, which are limestone. That distinction is the whole story, because the caves are in the limestone. Mount Api carries the famous limestone pinnacles on its upper slopes, and the Melinau Gorge separates Benarat from Api.

The geology is a metamorphosed sedimentary sequence of Palaeocene to Miocene age, and the surface above the caves runs to gorges, karst towers, terraces, hot springs, floodplains and waterfalls.

The Sarawak Chamber itself is reached only on a demanding guided caving trip, and even inside it a torch beam will not reach the far wall.

The park is a UNESCO World Heritage Site, and what is above the caves is as unusual as what is below. Mount Mulu’s summit carries moss forest, and the upper slopes of Mount Api hold the limestone pinnacles, blades of rock standing clear of the forest canopy.