Neb. Rev. Stat. § 57-208

When an oil, gas or mineral lease is given on land situated within the State of Nebraska, the recording thereof in the office of the register of deeds of the county in which the land is located shall impart notice to the public of the validity and continuance of such lease for the definite term therein expressed, but no longer; Provided, that if such lease contains the statement of any contingency upon the happening of which the term of any such lease may be extended, such as and as much longer as oil and gas or either is produced in paying quantities, the owner of such lease may at any time before the expiration of the definite term of the lease file with the register of deeds an affidavit setting forth the description of the lease, that the affiant is the owner thereof and the facts showing that the required contingency has happened. This affidavit shall be recorded in full by the register of deeds, and such record together with that of the lease shall be due notice to the public of the existence and continuing validity of such lease, until the same shall be forfeited, canceled, set aside or surrendered according to law.