File:Rotunda (Mammoth Cave, Kentucky, USA) 5 (51283584052).jpg

(looking southeast) (photo by Ljubomir Risteski)

The Rotunda (also known as the Vestibule) is the first room encountered after entering the Historic Entrance in Kentucky's Mammoth Cave, the longest cave on Earth. 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, Broadway Avenue (= shadowed area at left), and Audubon Avenue.

The dust-covered area behind by railing has wooden artifacts from an 1810s saltpetre mining operation. 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 (from 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 photographer). 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). A thin interval of dark, recessive-weathering, argillaceous limestone occurs near the ceiling - the Bethel Member of the Girkin Limestone. The Rotunda ceiling has gray limestones of the lower Beaver Bend Member of the Girkin Limestone.

The circular ceiling of the Rotunda (see elsewhere in this photo album) is a breakout dome.