A limestone cave or cavern is a natural cavity that is formed underneath the Earth’s surface that can range from a few metres to many kilometres in length and depth.
Most of the world’s caves, including those at the Cradle of Humankind, are formed in porous limestone. Over millions of years, acidic groundwater or underground rivers dissolve away the limestone, leaving cavities which grow over time.
Early life forms appeared in the oceans about 3.8-billion years ago. These were single-celled, blue-green algae, called cyanobacteria, which made their own food through photosynthesis, releasing oxygen into the atmosphere in the process.
Dolomitic limestone, a sedimentary rock, was formed over millions of years through chemical reactions generated by these early organisms.
With movements within the Earth’s crust, the sedimentary dolomitic limestone eventually became exposed on dry land.
As time passed the limestone, which is permeable and soluble, was eroded by water. Weak carbonic acid in rainwater, reacting with the chemicals in the rock, dissolved and eroded away the limestone as the water filtered into the underlying depths of sediments. Large hollow solution cavities were formed in the limestone in this way.
Many cavities occur at various depths in a cave system due to the continual seepage and flow of the mildly acidic water through the deposits, while underground rivers may eventually carve their way through a mountainside, creating openings and entrances to the outside.