Colossal Cave is located in the Escabrosa Limestone of Mississippian origin 300 million years old. The limestone was formed in a great inland sea of that period from the calcium deposits laid down by the remains of a sea creature called the crinoid, a small plant-like sea animal. The mountains in which Colossal Cave is formed were a mid-tertiary development caused by a crustal upheaval of the earth approximately 40 million years ago. The cave system was carved out of the solid limestone by the action of seeping water that eventually formed an underground river with many intricate tributaries. The full extent of the Cave is not yet known but explorations have reached a length of 39 miles. The many weird and beautiful crystal formations that cover the interior of the cave were formed over a period of thens of thousands of years. The number of different crystal formations are astronomical and their sizes vary from the minute to the colossal.
Bob Petley photos
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This was written several years ago, so I wanted to find out how long they found out the cave was.
On the official park website it reads:
Colossal Cave has never been fully explored. Although there are an estimated 39 miles of cave tunnels.
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So it looks like they never found more tunnels since back in the 1970's.