English: Western Kentucky's Mammoth Cave is the longest cave system in the world. As of fall 2022, 426 miles of cave passages have been mapped. Seen here is a small passage in the floor of 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, a subterranean river used to flow through here, but 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. Cleaveland Avenue is at Level C in the Mammoth Cave system. Level C passages started forming about 1.9 million years ago. Cleaveland Avenue is developed in Mississippian-aged limestones.

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.

Cleaveland Avenue is accessible to modern tourists on several tours. A famous one is the "Wild Cave Tour" that is about 6 hours and 6 miles. The tour involves upright walking, climbing, knee crawling, belly crawling, and canyon walking. Seen here is a park ranger entering a small passage on the floor of Cleaveland Avenue. This is the "Birth Canal", which is often a part of the Wild Cave Tour.