What is a mountain man? Why were they significant?

We have 2 tenses here. "what is" "why were". Most present mountain men today are just escapists. Early in the formation of this country, mountain men like Boone, Crockett, Lewis and Clark and others, explored and helped settle the nation. They had vision and asked questions. If everybody today still believed the world was flat, you'd be living in Europe or Africa, be taught that the earth was the center of the universe and the ocean was full of sea monsters.