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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 lime 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 eaves 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 of 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 look 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 

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