EducationAisleI think in the question a bunch of people seem to miss the point. Can you let me know if my understanding is okay?
A successful ascent of Mt. Everest is rendered tremendously difficult by
numerous obstacles,
not the least of which is the challenge of adjusting to the decreased atmospheric pressure and correspondingly low oxygen availability that characterize very high altitudes.
Lets first consider this sentence
The new models explained alot of sub-atomic particles, some of which were recently discovered
Whats the subject of were? subatomic particles! Since some take the singular/plural property of what follows and which here refers to sub-atomic particles.
Coming back to our sentence
difficult by
numerous obstacles, [i]not the least of which is the challenge of adjusting ...
what does which refer to? Numerous obstacles. The sentence structure is not inverted or anything. The reason why this is singular is because of the word
not the least . Just like One of the oranges is singular. not the least of numerous obstacles is also singular, and hence the requirement for singular verb.