Archeologists on a mission to see where a mysterious passageway under the world's longest known cave system goes are uncovering 19th and 20th-century artifacts in the historical dig.
This collapsed sinkhole, the largest known natural entrance to the Mammoth Cave system, is believed by researchers to be where the cave's extensive labyrinth of naturally formed passages first began at Mammoth Cave National Park in Kentucky.
Archeologists from the National Park Service and the University of Idaho found artifacts - including coke bottles, ticket stubs, ceramic pieces and other items from over 200 years ago, according to Bowling Green Daily news.
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Water cascading from the limestone layers at the entrance - which has been used for over 5,000 years - has revealed more features inside the pit.
Kailey Alessi, a University of Idaho master's student researching the cave, told the Bowling Green Daily News that researchers found evidence of prehistoric peoples who lit fires in the cave.
Historians believe a 50 feet vertical pit directly below the waterfall at Mammoth Cave's entrance was filled around the 1810s during the saltpetre mining operation, according to a statement on the park's Facebook page. The assumption is based on a historical map from 1835 identifying a passage as "filled."
"This project will provide researchers and scientists with even more information about the cultural and natural resources of Mammoth Cave and help us learn how to better protect and interpret the park for you and future generations," the Facebook post read.
Significant discoveries in geology, hydrology, speleology, archaeology, biology, and microbiology have come from research at Mammoth Cave, according to National Park Service.
Camille Fine is a trending visual producer on USA TODAY's NOW team.
This article originally appeared on USA TODAY: Mammoth Cave historical dig uncovers 19th and 20th-century artifacts