On the island of Palawan in the Philippines is a layer of limestone over 500 metres (1,640 feet) thick. The rock is honeycombed with a complex network of caves – some big enough to hold jumbo jets – that have formed due to running water from rain and streams. Deep inside the limestone is the Puerto Princesa Subterranean River, which flows 8.2 kilometres (five miles) through a warren of passages to the sea.
Underground rivers like the Puerto Princesa are found worldwide in a type of limestone terrain called karst. These dramatic landscapes are riddled with huge caves, pits and gorges. Famous examples include the South China Karst, which covers 500,000 square kilometres (193,000 square miles) of China’s Yunnan, Guizhou and Guangxi provinces.
Karst forms when acid water seeps down tiny cracks, called joints, in the limestone. The acid slowly eats away the rock and enlarges the joints into vertical shafts and horizontal passages. Rivers flowing onto limestone often vanish from the surface down shafts called swallow holes and continue as underground waterways. Generally, dry valleys signal where the river once flowed on the surface.
Over millions of years, underground rivers can carve out huge cave networks – some that extend for hundreds of kilometres. Higher caves are left abandoned when gravity causes the river to drain into lower passages. The water seeps down through the limestone until it reaches impermeable rocks, then flows horizontally until it emerges near the base of the karst as a spring or waterfall.
During floods, or when the water table rises, the river can totally fill a cave and erode its roof. When the water retreats, the unsupported ceiling may crumble. The Reka Valley in Slovenia – a 100-metre (328-foot)-high gorge – formed when a cave collapsed centuries ago. This means the Reka River, which primarily runs underground through the Škocjan Caves, now sees daylight for part of its journey.