The Green River and Cave Country
Near Mammoth Cave, Kentucky
The quiet river winding through these hills is the sculptor of everything underground. This is the Green River, and it is the reason the world's longest cave lies right here and not somewhere else. As the Green River slowly cut its valley deeper over millions of years, it lowered the level to which groundwater could drain.
Every time the river dropped, the water inside the limestone found a new floor to carve, stacking cave passages on top of one another like the levels of a parking garage, several stories of them in places. Rainwater soaking into the surface turns faintly acidic, sinks through countless cracks and sinkholes, and dissolves the soft limestone grain by grain until it reaches the river. That is the recipe for karst country, a landscape of disappearing streams, springs, and hidden hollows, and this valley is a classic example of it.
The Green River still flows through the national park today, home to a rich variety of fish and mussels. Above it spreads a forest of oak and hickory, and below it runs a labyrinth that people have explored for five thousand years and still have not reached the end of.
