Lewis and Clark Caverns State Park - Whitehall MT Big Timber Pioneer reported in 1936 that a CCC camp at what was then known as "Morrison Cave" built "a road to the mouth of the cave" and conducted "exploratory work inside of the cavern." The CCC did much of the work to make the cave system accessible to visitors.
Wikipedia explains more of the park's history:
“Lewis and Clark Caverns State Park is located in southeastern Jefferson County, Montana. The primary feature of the park is its namesake cavern.
The cavern was discovered in 1892 by local ranchers Tom Williams and Bert Pannel. The cave was first developed for tours by...