Fossils of prehistoric sharks have been found in Mammoth Cave, with some reports suggesting the discovery includes fossils of a giant shark species. The cave system, which may be the world's longest, has yielded fossils of various ancient species, including sharks that lived around 337 million years ago. The number of shark species identified from the fossils is reported to be at least 40, although some outlets mention specific discoveries, such as a 340-million-year-old shark species and fossils of megalodons.