Earth

Some chemistry

Most of the longest and deepest caves in the world are formed by chemical corrosion processes in rocks particularly water-soluble thanks to their mineral composition. These corrosion processes are known as karst processes.
Water and rock
All minerals are more or less soluble in water, but some are much more soluble than others, and require a very short time span to dissolve (in a geological sense, naturally), while others require much longer periods of time, and are therefore considered practically insoluble. Rocks made of the most soluble minerals are the ones in which karst features develop more easily, even though the karst process is a complex one in which rock composition is only one of the many factors involved in the development of the phenomenon.
When studying the solubility of the main minerals forming the most diffused rocks of the Earth’s surface, it can be observed how the solubility of the different minerals varies greatly. For this reason, rocks such as rock salt (made of sodium chloride NaCl, or halite), one of the most water-soluble minerals, are practically absent in humid climates as they dissolve rapidly. In quartzite rocks, made of quartz (SiO₂), one of the minerals most resistant to weathering,  karst features may develop only in particular climatic conditions and in areas in which waters had a very long time, i.e. millions of years, to dissolve the rocks (e.g. the Amazon Tepuy quartzite cave systems ). 
Not only water
In nature, however, things are not that simple. In fact,  natural waters are never pure waters, but they are water solutions containing various ions dissolved within them that can increase the aggressiveness and corrosive action on some types of rocks, thus complicating the simple dissolution reaction. The process is well known to those who have to clean the bathroom at home. In order to remove the calcareous incrustations that ruin all the bathroom fittings (geologically speaking, these are calcium carbonate crystals, CaCO₃, calcite; rocks that prevalently contain calcite are called limestones), we use aqueous solutions enriched with acids that increase their corrosive streght, such as hydrochloric acid (also known to housewives as muriatic acid), or acetic acid, that are present in many house-cleaning products. These substances make the removal of incrustations easier in two ways: on one hand they increase the solubility of calcite, on the other they greatly accelerate the  speed of reaction (that is very rapid and violent, in fact bubbles form because of gases freed when these products are used). Even pure water could obtain the same results, but the time taken would be decidedly beyond the scale of human observation…. and by far too long for the housewives’ patience! In fact natural water behaves in the same way as the detergents, but since the acid solution is much more diluted and with much weaker acids, chemical reactions are much slower, at least on a human-observation scale. Nature, unlike housewives, has no hurry, and the results are even more spectacular!