Sentence:  It is more than the story of the fence and those who maintain it. It is a journey not just into the inland and along the length of the fence; it is a journey into the Australian psyche, reminding us of the <WSD>inexorable</WSD> struggle against the implacable wilderness, of the thin line that separates the tamed from the untamed, and the unnamed menace that broods just beyond the last suburbs. 
Meaning of the word marked with <WSD> tags:  Impossible to prevent or stop; inevitable. (from mid 16th c.)