1If there is one natural feature which has long fascinated man, it is caves
2those natural underground spaces into which people can enter.
3No known cave in the world can exist without it being extensively explored,
4whether it be in remote areas, such as the Clearwater Cave in Borneo,
5or immensely long and difficult, such as Mammoth Cave in America,
6or perhaps the most challenging: those caves below the water table, full with water,
7in which case the exploration is known as cave diving.
8Whatever the case, if the caves are known,
9as complex and difficult as they usually are, they will be explored.
10The formal name for the study of caves is speleology,
11and involves many disciplines, such as chemistry,
12geology, biology, and cartography,
13or the science of making accurate maps,
14since the largest caves can be hundreds of kilometers long and highly complicated.
15If the exploration is just for fun, the activity is known as caving,
16but with the difficulties and dangers involved,
17the average caver is often involved with one of these speleological sciences.
18As for the number of caves in the world,
19research suggests that only a fraction have been found and documented,
20mostly in the areas in which caving has long been a popular activity,
21such as in America, France, or Australia.
22China, for example, has huge areas of limestone bedrock,
23prime material for cave formation,
24and logically must contain among the largest number of caves in the world,
25yet only few are documented due to the lack of interest in caving there.
26Thus, as exploration continues, new caves will be discovered,
27and it is likely that the numbers could radically change in the future.
28With caves being such mysterious and intriguing places,
29the obvious question concerns how they form.
30The most common cause is the effect of slightly acidic water
31in a process called 'dissolution'.
32Here, water seeps into the ground, and down through rock masses.
33If this rock is soluble, the water dissolves some of it,
34and over time, the passage expands to become a cave, or cave system.
35The largest and most common of these involves limestone,
36although other materials are possible, including gypsum, marble, and even salt.
37Now, if the cave is formed at the same time as the rock, it is called a primary cave,
38and the most common of these are lava tubes.
39When lava from volcanoes flows downhill,
40the topmost surface cools and solidifies first,
41leaving the hotter lava to continue to flow beneath.
42If most of this eventually flows out, a hollow tube is left.
43The country Iceland, has some excellent caves of this type,
44and even far-flung Australia has a few.
45Finally, there are sea caves, formed from waves pounding into coastal cliffs,
46and eroding into fault lines, or softer rock.
47These caves are usually the shortest, about 50 meters at most,
48since the erosive action of the waves can only reach a certain distance.
49However, it is limestone caves which will always be the most interesting.
50This is due to the presence of limestone formations.
51As the groundwater seeps downward through the ceiling of the caves,
52it encounters a different pressure and temperature,
53causing it to deposit a small amount of its dissolved calcium carbonate.
54Over time, through the action of millions of drops,
55this deposition process results in ever-growing masses,
56known as stalactites (if hanging from the ceiling),
57or stalagmites (if growing from the floor).
58Alternatively, the water may flow over a large surface of rock, forming flowstone,
59and with other sorts of processes,
60can result in beautiful milky-white formations of astonishing complexity.
61One of the best examples of this,
62as well as of what surprising discoveries may yet await,
63is the Lechuguilla Cave in New Mexico.
64In the 1950s, some cavers heard the sound of wind underneath some large boulders on the cave floor,
65a clear sign of a deeper passageway.
66It took until 1984 for a group of cavers to gain permission to start digging,
67and two years later, in 1986,
68they broke through into a walkable passageway,
69unlocking one of the longest cave systems in the world,
70the deepest in America,
71and some of the most beautiful formations ever found.
72Unusually, many of these were yellow with sulphur,
73suggesting that the caves were a result of hydrogen-sulfide from nearby oil deposits.
74This was forced through fracture lines,
75combining with existing groundwater to form sulfuric acid,
76dissolving the limestone from the bottom up,
77instead of the normal top-down seepage which characterises most limestone cave formations.