To make a cave you need rock that will dissolve, and a way for water to flow through the rock, Joe Meiman, a park hydrologist, explains. Once water enters the rock, it wends its way through crevices in the soluble limestone, slowly enlarging tiny fractures into an underground riverbed. After at least 2 million years of water flow, the large caverns of the uppermost layer of the cave were formed, according to a new dating technique based on radioactive decay in quartz developed by Darryl Granger, a geology professor at Purdue University in Indiana. Then the water table shifted downward, and began forming another level of the cave beneath the first. Mammoth Cave has four major levels, each descending deeper into the earth.
Water runs through the rock beneath, where the cave is still forming.
A cap of more slowly eroding sandstone protects the upper levels of the cave from wearing away themselves, helping make this the largest recorded cave system in the world. My grandchildren will still be finding cave out here. Over 350 miles of cave have been mapped so far, and some speculate the cave could have over 600 miles of corridors. While hundreds of people wander this vast cave on tours, Meiman often works on his research in silence, just miles away in the same cave network. But the cave is not totally silent.