The meaning of David Grann's statement that the Osage had the first-ever "underground reservation" is that the Osage tribe was able to secure the head-rights in the mineral resources in their territory through careful and visionary negotiations with the U.S. Government.
Thus, David Grann could rightly say that the Osage had the first-ever "underground reservation" in his "Killers of the Flower Moon" because of the Osage's share in the tribe’s mineral trust that they secured for themselves.
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