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Image rights
The Bottomless Pit in Mammoth Cave, woodcut, 1887 (Nuno Carvalho de Sousa Collection, Lisbon). Dr. Nuno Carvalho de Sousa Private Collections - Lisbon Date: 1887 Type: Woodcutprint. Accessed through WikiCommons. This image (or other media file) is in the public domain because its copyright has expired.