30 U.S.C. § 28b
The performance of not less than $100 worth of labor or the making of improvements aggregating such amount, which labor or improvements are required under the provisions of section 28 of this title to be made during each year, may be deferred by the Secretary of the Interior as to any mining claim or group of claims in the United States upon the submission by the claimant of evidence satisfactory to the Secretary that such mining claim or group of claims is surrounded by lands over which a right-of-way for the performance of such assessment work has been denied or is in litigation or is in the process of acquisition under State law or that other legal impediments exist which affect the right of the claimant to enter upon the surface of such claim or group of claims or to gain access to the boundaries thereof.
30 U.S.C. § 28b