Rainforest rain and groundwater adds to the ‘diversity’ of cave systems. Limestone rock erodes easily due to the porosity and softness of calcium carbonate and the process of dissolution into acid. Weak carbonic acid forms between water and carbon dioxide and rainwater already slightly acidic passes through the atmosphere and soil layer ~ mixes with CO2 and forms carbonic acid. Acidic water passes through fractures ~ crevices and cavities and further dissolves limestone ~ enlarging the network of passageways. These “solutional” caves ~ usually take more than 100,000 years to grow large enough for a human to be able to pass through.