The Jewel Cave is one of the longest caves in the world and over the past two months, volunteer explorers have found it's longer than previously thought.
The National Park Service says the known length has been extended 13 miles from 177 to 180 miles long.
Two small lakes have also been discovered.
It’s the third longest cave in the world and home to nine bat species.
The cave is only explored by trained volunteers and thanks to airflow studies, it's thought to be even longer.
“Airflow within the cave is the single best indicator that large areas of the cave are yet to be discovered,” the National Park Service writes on its website.
The discovery trips can last anywhere from six to 18 hours.
The Jewel Cave National Monument is located 13 miles west of Custer.
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