| If there is one natural feature which has long fascinated man, it is caves |
| those natural underground spaces into which people can enter. |
| No known cave in the world can exist without it being extensively explored, |
| whether it be in remote areas, such as the Clearwater Cave in Borneo, |
| or immensely long and difficult, such as Mammoth Cave in America, |
| or perhaps the most challenging: those caves below the water table, full with water, |
| in which case the exploration is known as cave diving. |
| Whatever the case, if the caves are known, |
| as complex and difficult as they usually are, they will be explored. |
| The formal name for the study of caves is speleology, |
| and involves many disciplines, such as chemistry, |
| geology, biology, and cartography, |
| or the science of making accurate maps, |
| since the largest caves can be hundreds of kilometers long and highly complicated. |
| If the exploration is just for fun, the activity is known as caving, |
| but with the difficulties and dangers involved, |
| the average caver is often involved with one of these speleological sciences. |
| As for the number of caves in the world, |
| research suggests that only a fraction have been found and documented, |
| mostly in the areas in which caving has long been a popular activity, |
| such as in America, France, or Australia. |
| China, for example, has huge areas of limestone bedrock, |
| prime material for cave formation, |
| and logically must contain among the largest number of caves in the world, |
| yet only few are documented due to the lack of interest in caving there. |
| Thus, as exploration continues, new caves will be discovered, |
| and it is likely that the numbers could radically change in the future. |
| With caves being such mysterious and intriguing places, |
| the obvious question concerns how they form. |
| The most common cause is the effect of slightly acidic water |
| in a process called 'dissolution'. |
| Here, water seeps into the ground, and down through rock masses. |
| If this rock is soluble, the water dissolves some of it, |
| and over time, the passage expands to become a cave, or cave system. |
| The largest and most common of these involves limestone, |
| although other materials are possible, including gypsum, marble, and even salt. |
| Now, if the cave is formed at the same time as the rock, it is called a primary cave, |
| and the most common of these are lava tubes. |
| When lava from volcanoes flows downhill, |
| the topmost surface cools and solidifies first, |
| leaving the hotter lava to continue to flow beneath. |
| If most of this eventually flows out, a hollow tube is left. |
| The country Iceland, has some excellent caves of this type, |
| and even far-flung Australia has a few. |
| Finally, there are sea caves, formed from waves pounding into coastal cliffs, |
| and eroding into fault lines, or softer rock. |
| These caves are usually the shortest, about 50 meters at most, |
| since the erosive action of the waves can only reach a certain distance. |
| However, it is limestone caves which will always be the most interesting. |
| This is due to the presence of limestone formations. |
| As the groundwater seeps downward through the ceiling of the caves, |
| it encounters a different pressure and temperature, |
| causing it to deposit a small amount of its dissolved calcium carbonate. |
| Over time, through the action of millions of drops, |
| this deposition process results in ever-growing masses, |
| known as stalactites (if hanging from the ceiling), |
| or stalagmites (if growing from the floor). |
| Alternatively, the water may flow over a large surface of rock, forming flowstone, |
| and with other sorts of processes, |
| can result in beautiful milky-white formations of astonishing complexity. |
| One of the best examples of this, |
| as well as of what surprising discoveries may yet await, |
| is the Lechuguilla Cave in New Mexico. |
| In the 1950s, some cavers heard the sound of wind underneath some large boulders on the cave floor, |
| a clear sign of a deeper passageway. |
| It took until 1984 for a group of cavers to gain permission to start digging, |
| and two years later, in 1986, |
| they broke through into a walkable passageway, |
| unlocking one of the longest cave systems in the world, |
| the deepest in America, |
| and some of the most beautiful formations ever found. |
| Unusually, many of these were yellow with sulphur, |
| suggesting that the caves were a result of hydrogen-sulfide from nearby oil deposits. |
| This was forced through fracture lines, |
| combining with existing groundwater to form sulfuric acid, |
| dissolving the limestone from the bottom up, |
| instead of the normal top-down seepage which characterises most limestone cave formations. |