Beneath the rolling hills of south-central Kentucky, subterranean chambers coalesce over 400 square miles to form the largest cave system in the world, the aptly named Mammoth Cave.

The vastness of Mammoth Cave belies its formation by literally one drop of water at a time.

Groundwater percolating through the fractured limestone bedrock dissolves the rock a molecule at a time, slowly enlarging the fractures.

Once a cave is formed, as mineral-laden groundwater drips down from the roof of the cave, crystals of calcium carbonate precipitate out and adhere to the cave roof—the beginning of a stalactite, icicle-like formations that hold tight to the ceiling. On the cave floor beneath the dripping water, calcium carbonate crystals may accumulate in a stalagmite; remember it just might reach the cave roof.

More info on Mammoth Cave National park here.

Photo from here.