Western Kentucky's Mammoth Cave is the longest cave system on Earth, with 412 miles known and mapped as of fall 2017. The name does not refer to the early discovery of fossil mastodon or mammoth bones here. Rather, the name refers to the immense size of many rooms and passages.

Shown here is a medium-sized, actively-forming vertical shaft called "Water Clock". It is located along the northern wall of Main Cave, the principal trunk passage in the Mammoth Cave Ridge portion of the system. The rocks are gray marine limestones in the upper Ste. Genevieve Limestone (upper Middle Mississippian). Vertical shafts are dissolutional cave passages formed in the vadose zone - above the water table. Slightly acidic, downward-dripping water partially dissolves limestone, resulting in a silo-shaped to irregularly-shaped, vertically-oriented passage. Large, well-formed vertical shafts are called domepits. Well known examples of the latter in Mammoth Cave include Sidesaddle Pit, Bottomless Pit, Mammoth Dome, and Cathedral Dome. Water Clock gets its name from the steady, regular sound of falling water drips. Reportedly, the beat does not change during outside storms.

Main Cave is at level B in the Mammoth Cave system, which is the 2nd-oldest and the 2nd-elevationally highest set of passages. Level B passages started forming during the Pliocene, 2 to 4 million years ago. A subterranean river used to flow through Main Cave, but the water table dropped long ago and the passage is now dry. Water Clock is an actively-forming feature that is much younger than the Main Cave passage it has intersected.