Limestone is the most common sedimentary rock in the world and it is also the rock that is best suited to large cave formations. Of all the types of caves in the world it is the limestone caves that are the most extensive and beautiful. This has made them tourist attractions throughout the world. They are available for adventurers or the everyday family to explore and wonder about.
The majority of limestone is made up of calcite, also called calcium carbonate. Originally, limestone was created at the bottom of the ocean floor millions of years ago. The calcite came from the shells of marine organisms and over time it created the limestone we know today. Limestone is considered highly soluble, meaning that it can be easily dissolved.
Limestone caves are formed over hundreds, thousands or even millions of years. Similarly to how a canyon above the ground is formed, water plays a specific role in the formation of caves. Limestone is a curious type of rock in that, while it may be easily eroded, it cannot be eroded by water alone. A pure running stream would not create a limestone cave the same way that it would a canyon of another type of rock. The only kind of water that will easily erode limestone and create a cave, is rain water.
The reason for this is surprisingly simple. Rain water does not stay pure on its journey through the air and to the ground. Along its path the rain water will pick up carbon dioxide from the air that it passes through and bring that down to the ground with it. As the rain water seeps through the dirt and upper soil, if continually picks up more carbon dioxide from decomposing plants that are beneath the surface. Once the rain water hits the limestone it creates a chemical reaction with the calcite and causes small portions of it to erode and be washed away. With enough time and enough rain a limestone cave will slowly start to form.
As the calcite rich water continues to move through the cave, it can be deposited into other areas which is how we get formations such as stalagmites. There is only one other known method that can form a limestone cave at this current time. Recently it was discovered that several limestone caverns, including Carlsbad Caverns, were formed by sulfuric acid. They discovered a micro-organism that consumes underground oil and releases sulfuric acid. This acid traveled through the rock upward and eroded the limestone above them to create caves. This is a more unusual method of limestone cave formation and the vast majority is through erosion from rain water.
There is also evidence that even after the cave is formed, erosion continues to happen indefinitely. Rain water creates caves are often very wet, sometimes even having small streams running through them. This is evidence that water still has access to the cave like it has throughout its formation. It has been evidenced in many caves that the water that flows out of the cave contains a much higher level of calcite than when it entered the cave. This is evidence that the cave is still eroding and will continue to change form as time goes on.
If one would like to view some of these amazing geological formations, there are over 130 cave systems open to the public in the United States alone. There are caves available to view in many other countries as well, though perhaps not as extensively as the United States. For bible scholars, there are some of the cave systems believed to be mentioned in the Bible available for public as well. Everyone should take a few moments to tour one of these wonders of nature, it is truly a sight that cannot be seen anywhere else.