Also known as: Colossal Cave Entrance, E-19 Mammoth Cave National Park, Mammoth Cave, Kentucky
Photo
Colossal Cavern Entrance
By National Park Service - http://www.hscl.cr.nps.gov/insidenps/report.asp?STATE=KY&PARK=MACA&STRUCTURE=&SORT=3&RECORDNO=10, Public Domain, https://commons.wikimedia.org/w/index.php?curid=8921182