(public domain photo provided by the American National Park Service)

The Rotunda (also known as the Vestibule) is the first cave room encountered after entering Mammoth Cave's Historic Entrance. The Rotunda is an impressively large cave room having a rounded shape - this is best seen in the ceiling. The Rotunda is at the intersection of three passages: Houchins Narrows (behind the photographer here), Broadway Avenue (off to the left in the background), and Audubon Avenue (off to the right in the background).

Wooden artifacts from an 1810s saltpetre mining operation are present in the room - these remains principally consist of old leaching vats. During wintertime in the past, cave visitors could witness thousands of hibernating bats on the walls and ceiling of the Rotunda. Paleontologic evidence of bats in Mammoth Cave also suggests that former bat populations were likely in the millions.

The Rotunda and the three passages extending from it are part of Mammoth Cave's level B, the 2nd-highest level of passages, formed before 3.25 million years ago (Late Pliocene or before). When level B was flooded, water flowed from Broadway Avenue (off to the left) into the Rotunda, and then into Audubon Avenue (off to the right). For a while, water also flowed into Houchins Narrows (behind the viewer). Level B is currently well above present water table. The Rotunda and the three passages extending from it are now dry.

Most of the bedrock in the walls of the Rotunda Room consists of gray limestones of the Paoli Member of the Girkin Limestone (lower Chesterian Series, lower Upper Mississippian). The Rotunda ceiling has gray limestones of the lower Beaver Bend Member of the Girkin Limestone.

The circular ceiling of the Rotunda itself is a breakout dome.

Seen here is the Rotunda on 13 June 2021 with natural fog, a rare meteorological event in Mammoth Cave. Park personnel noted that a surface storm resulted in an air temperature drop and an air pressure increase outside. Under normal conditions in summertime, cave air flows outward from the cave, and in wintertime, air flows inward. The storm-induced air pressure change here reversed airflow - humid air rushed in from the entrance, through Houchins Narrows, into the Rotunda area, and fog formed. Reports indicate that the fog dissipated after about 10 minutes.