It's part of the standard nomenclature, and has its origins in mountaineering - there's a point climbing a mountain where the altitude is thin enough that there

It's part of the standard nomenclature, and has its origins in mountaineering - there's a point climbing a mountain where the altitude is thin enough that there's not enough atmospheric oxygen to sustain human life. The idea of an environment at the heights of a webpage that's inimical to "untrusted" life is a good analogy, in my opinion. And it has the benefit of being dramatic and attention grabby! *Edited for grammar and clarity