30 U.S.C. § 121 — Agricultural entry or purchase of lands withdrawn or classified as containing phosphate, nitrate, potash, oil, or gas; reservations to United States; application

This section covers public land that has been withdrawn from general use, or classified, because it contains — or might contain — phosphate, nitrate, potash, oil, gas, or asphalt-related minerals. People can still claim, select, file an entry on, or purchase that land under the ordinary non-mineral public land laws, as long as the land is otherwise available. But this only works if the claim is made on the understanding that the United States keeps ("reserves") ownership of the mineral deposits that caused the land to be withdrawn, classified, or reported as valuable — along with the right to prospect for, mine, and remove those minerals later. Every application to locate, select, enter, or purchase this kind of land must state that it's made under, and subject to, the rules and reservations set out in sections 121 through 123 of this title.