A cave also known as a cavern is an underground space which is large enough for a human to enter it. These are natural space i.e. they are formed naturally by the process of weathering of rocks and they extend deep in the ground.
There are a varied number of geological processes which cause the formation of caves; these may involve any combination of water erosion, tectonic forces, chemical processes, microorganisms, atmospheric influences and pressure. Based on the process causing the formation there are different cave types like the solution cave, primary cave, sea cave, glacier cave etc.
There are numerous caves that spread all around the world and date back to ancient times. It may not always be possible to determine the time when the caves where formed. There is a list of some oldest caves in the world based on the year of their discovery.
The oldest stated caves in the world are the Mammoth Cave situated in Brownsville, Kentucky, USA; the caves were discovered in 1791. The Holloch in Canton of Schwyz, Switzerland are the second oldest caves discovered in the year 1875.
The other caves on the list of oldest caves are Wind Cave (USA, 1881), Jewel Cave (USA, 1900), Optymistychna Cave (Ukraine, 1966) and the Gua Air Jernih (Malaysia, 19780) amongst many other.