16 U.S.C. § 404b-2
The Secretary of the Interior is authorized to cooperate with the State of Kentucky for the purpose of arranging for the eventual acquisition by the United States of the Great Onyx Cave and the Crystal Cave within the authorized boundaries of Mammoth Cave National Park. The Secretary shall deposit to the credit of a special receipt account that portion of the annual admission, guide, and elevator fee receipts from the said park which exceeds the annual amount available to the park for management, guide, and protection purposes, which funds so deposited may be expended thereafter in payment for the purchase of said cave properties. The Secretary is further authorized to enter into such contracts and agreements as he may determine to be necessary to effectuate the acquisition of the cave properties as authorized herein.
16 U.S.C. § 404b-2