Hoefler

Plaintiffs appeal from the district court’s grant of summary judgment and dismissal of their claims relating to a decision of the Interior Board of Land Appeals (“IBLA”), which affirmed a decision of the Bureau of Land Management, declaring their mining claim void ab initio. We have jurisdiction pursuant to 28 U.S.C. § 1291 and we affirm.

I.

We review de novo a grant of summary judgment, Forsyth v. Humana, Inc., 114 F.3d 1467, 1474 (9th Cir.), cert. denied, — U.S. -, 118 S.Ct. 559 , 139 L.Ed.2d 401 (1997), as well as a dismissal for failure to state a claim. Lewis v. Telephone Employees Credit Union, 87 F.3d 1537, 1545 (9th Cir.1996). The existence of subject matter jurisdiction is a question of law reviewed de novo. Wilson v. A.H. Belo Corp., 87 F.3d 393, 396 (9th Cir.1996). We review decisions of the IBLA under the arbitrary and capricious standard of the Administrative Procedure Act (“APA”). 5 U.S.C. § 706 (2)(A); Gilmore v. Lujan, 947 F.2d 1409, 1411 (9th Cir.1991). Because estoppel is an equitable concept that is invoked by the court in its discretion, we review the district court’s rejection of appellants’ equitable estoppel argument under the abuse of discretion…