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Stalactites and stalagmites in a limestone caveCaves are underground holes in the rock. Some caves open up when the ground splits, as in an earthquake. Some are eroded by waves hurling stones and pebbles at a cliff. But most are made in limestone, a kind of sedimentary rock, by a chemical process. Rainwater naturally contains tiny amounts of acid. It trickles into cracks and reacts with the rock made of soluble calcium carbonate to dissolve it away, little by little. Over thousands of years, the small cracks are gradually widened into huge caves.

Limestone caves

An underground stream flows through limestone rock, creating a cave system.Limestone is easily dissolved by rainwater, which is actually a very weak acid. It seeps through the soil and into cracks in the rock, slowly eroding it away. Streams and rivers, too, easily wear away limestone and even carve channels for themselves in the rock deep underground. Over thousands of years, this water action forms caves, often made up of a series of chambers, tunnels and passages in the limestone, which can run for tens of kilometres underground.

Mammoth Cave in Kentucky, USA, is the longest known cave system in the world, extending more than 600 km (370 miles) underground. Sections of it are still unexplored. The cave may be linked to other nearby caves, extending its length to more than 800 km (500 miles).

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