Caves are formed by geologic processes. These may involve a combination of chemical processes, tectonic forces and atmospheric influences. Solutional caves may form anywhere with rock which is soluble, and are most prevalent in limestone, but can also form in other material, including chalk, dolomite, marble, loess, ice, granite, salt, lava, sandstone, and gypsum. The most common process of cave formation is karstification, which is the solution of rocks by rain water.
Cave formation in limestone occurs because limestone dissolves under the action of rainwater and groundwater charged with CO2 (carbonic acid) and naturally occurring organic acids.

The dissolution process produces a distinctive landform known as karst and characterized by sinkholes, sinking streams, and underground drainage. Limestone solution is the single most important process forming caves and the origin of the great majority of all caves on Earth. The reason for this abundance is the facts that limestone is so common and the slowness of the solution process. If it was faster, the lifespan of limestone caves would be much shorter and their number much lower.

Limestone caves are often adorned with calcium carbonate formations produced through slow precipitation, including the most common and well-known stalactites and stalagmites.

There are some incredible caves and cave systems throughout the world that will take your breath away and defy imagination. Their appearance differs according to the type of rock they are formed from – so Fingal Cave in Scotland, for example, with its towering staffs of black, boxy basalt standing up dead straight, will look very different to the Cuevas de Marmol (or Marble Cathedral) in Chile, whose walls and ceilings are composed of the smooth, flowing lines and curves of calcium carbonate worn down by the movement of waves from the lake.

Stone isn’t the only thing that can affect the appearance of caves. The Waitomo Caves in New Zealand are home to a species of gloworm which light up the walls and ceiling in order to attract their prey. The Italian island of Capri is known for its Grotto Azzura (or Blue Grotto), a half-flooded sea cavern. When the sunlight shines into the cave in early afternoon through an opening at water level, the blue is at its brightest, glowing quite spectacularly.

Some caves offer up dizzying statistics. The deepest cave, in Abkhazia, Georgia, discovered in 2001, is estimated to be the world’s deepest cave. This cave, known as the Krubera Cave (or Voronja Cave), is estimated to be 2,080 m deep (6,800 ft). To reach the final part of the cave at the bottom, you must travel down around 300 m (984 ft) underwater. In Mexico, the Cave of the Crystals was discovered in 2000. Here are found some of the largest cave crystals ever – up to 11 m (36 ft) in length and 4 m (10 ft) in diameter. They grew so large due to the extremely high temperatures inside the cave – up to 58 degrees Celsius (136 degrees Fahrenheit), which also means that people cannot survive inside the cave for long, even with protective clothing, and as a result much of the cave is still unexplored.

You might think that underground rivers are the stuff of legends, but caves like the Puerto Princesa in the Philippines and the Skocjan Cave in Slovenia prove they can exist in real life. The Puerto Princesa underground river, at 8.2 km (5 miles) is the world’s longest navigable underground river, while the Skocjan caves have walkways on high bridges that resemble the dwarf mines in the Lord of the Rings stories.

Other spectacular caves include the Cave of the Swallows (Mexico), an open air pit which has a freefall drop of up to 370 m (1,214 ft). Thrill-seekers abseil and even base jump from the mouth of this cave. There are caves in Thailand, Myanmar and Malaysia, for example, which contain Buddhist temples. Ice caves are found throughout the world, and some of the most impressive of these can be found in Russia (Kamchatka), Iceland (Mýrdalsjökull Glacier), Austria (Eisriesenwelt, the biggest ice cave in the world) and elsewhere.

And how could we forget, of course, the Lascaux Caves in Montignac, France? Discovered in the 1940s by a teenager, these caves were discovered to contain the now world-famous paleolithic cave paintings, dating back around 17,300 years.

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