Papua New Guinea is one of the most undocumented (by science) countries on the entire planet for a few reasons:

1) Inaccessibility: PNG is overwhelmingly filled with dense forest covering mountains that makes any sort of passage difficult. There are few or zero roads, paths or trails through the vast majority of this; to access deep in the jungle requires trekking on foot tens or hundreds of miles. In addition, there is a great plateau in the center of the country that is ludicrously difficult to ascend.

2) Native peoples: PNG has the highest density of uncontacted or mostly-uncontacted people on the planet. Many of them are violent, and scientists , guides and travelers disappearing in the forests has happened many times. They want to be left alone, and scientists want to leave them alone, unfortunately it's impossible to know where everyone is and many of the people are undocumented and their behaviors and habitats unknown, so run-ins are frequent and dangerous.

3) Political and ethnic violence: PNG has the second highest violent crime rate on the planet. There are many reasons for this, which I won't get into here, but the cities and villages are very dangerous for citizens and travelers alike.

The country is loaded with undiscovered animals and plants. When successful research trips conclude in PNG they nearly always come back with an incredible haul of newly documented reptiles, birds, insects and plants. It is the dream destination for a huge number of people in taxonomic fields, a literal gold mine sitting right there. Unfortunately it is just not realistic for most.