The lessee shall have thirty days from the termination date in which to remove all improvements, except buildings and structures, from the premises except as authorized by the state, leaving all existing development in good order and repair, and without unnecessarily hampering future development and operation of the lease premises. All such improvements remaining on the lease premises after thirty days, including the buildings and structures, shall become the property of the state: Provided, That the lessee may upon written request to the department be granted an extension of time where forces beyond the control of the lessee prevent removal of said improvements within thirty days. If the subsurface resource is exhausted, the lessee shall remove all improvements unless otherwise permitted by the department.