What's interesting about Papua New Guinea's linguistic diversity is how it correlates with its topography. Sometimes steep mountain ranges have effectively separated peoples from each-other for thousands of years, to the point that their languages sometimes became mutually unintelligible even if they were only separated by a very small distance. This phenomenon also occurs in other parts of the world (the Caucuses, the Himalayas), but TTBOMK nowhere else to the degree of Papua New Guinea.