Western Kentucky's Mammoth Cave is the longest cave system in the world. As of fall 2017, 412 miles worth of passages are known and mapped.

Mammoth Cave has relatively little speleothem ("cave formations"), which refers to all secondary mineral deposits in a cave. Most of the speleothem that is present is travertine (composed of calcite - CaCO3 - calcium carbonate) and gypsum (CaSO4·2H2O - hydrous calcium sulfate).

Shown above is gypsum speleothem in Cleaveland Avenue, a long tubular passage in Mammoth Cave Ridge. Tubular passages are wider than they are tall, and are phreatic in origin - they form at or below the water table. In this case, the water table has long since lowered and the passage is currently dry. Local base level is the Green River. All water-bearing passages in the Mammoth Cave system drain toward the Green River, along which water emerges at springs.

When first discovered, Cleaveland Avenue had abundant, complex, large gypsum structures - some the size of celery stalks. Naturally-detached speleothem littered the floor. The passage has been intensely vandalized by thousands of guides and tourists over the decades. Moderately impressive gypsum speleothem still remains, such as gypsum coatings, crusts, flowers, and helictites. The original color of most of the gypsum speleothem was bright white. Almost all of it is now stained or highlighted with dark gray coloration - this is from decades of lantern smoke. Some gypsum is orangish-brown in color (see above), due to minor iron oxide impurity.

The sulfur in the gypsum is derived from pyrite in an overlying unit. Downward-percolating water oxidized the pyrite (FeS2 - iron sulfide - "fool's gold") and produced sulfuric acid (H2SO4). The acid dissolved part of the limestone, which liberated calcium (Ca). In the presence of water, the calcium and the sulfate (SO4) produced gypsum.

The fibrous, outward-radiating, curved structures seen above are gypsum flowers.

Cleaveland Avenue is at Level C in the Mammoth Cave system. Level C passages started forming about 1.9 million years ago. A subterranean river used to flow through the passage.