File:Vertical shaft (Bottomless Pit, Black Snake Avenue, Mammoth Cave, Kentucky, USA) 2 (19550410184).jpg

Western Kentucky's Mammoth Cave is the longest cave system on Earth, with 412 miles known and mapped as of March 2019. 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.

The system has a variety of cave passages: tubular passages, canyon passages, vertical shafts (domepits), keyhole passages, and giant canyon passages.

Bottomless Pit is a deep vertical shaft that bottoms out well into the Middle Mississippian St. Louis Limestone (the lowest encountered stratigraphic unit in Mammoth Cave). Vertical shafts, also called domepits, are large, silo-shaped, dissolutional features in caves. They form in the vadose zone (above the water table) as downward-moving groundwater dissolves limestone by enlarging joints or joint intersections.

The vertical grooves along the limestone walls are flutes. The rocks are Fredonia Member limestones (Ste. Genevieve Limestone, upper Middle Mississippian). Bottomless Pit used to be a turn-around point for early tours. Later tours crawled along a narrow edge. In modern times, it is readily traversed via a metal trail bridge.

This vertical shaft does have a bottom, despite the name. Its rubble-covered floor closely coincides with the level of the River Styx and the Echo River (= modern water table; = local base level).