The horizontal network of a cave system forms along joints and 
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weaknesses in the rock. Carbonic acid (formed by carbon dioxide 
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dissolving in rainwater) attacks the calcite in limestone rocks, eventually 
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dissolving the rock. The rainwater then flows underground through 
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dissolved sink holes and corrodes a horizontal cavern system at the level 
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of the water table. Drainage is established when the water breaks through 
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to the surface, forming a resurgence spring. If the water table is later 
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lowered, another cave system develops at the new level. Thus the upper 
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caves become dry whereas the lower, more recent, caves are water-filled.