S.C. Code Ann. § 3-1-10

The jurisdiction of the State is hereby ceded to the United States over so much land as is necessary for the public purposes of the United States; provided, that the jurisdiction hereby ceded shall not vest until the United States shall have acquired the title to the lands by grant or deed from the owner thereof and the evidences thereof shall have been recorded in the office where, by law, the title to such land is recorded. The United States is to retain such jurisdiction so long as such lands shall be used for the purposes aforementioned and no longer.