For centuries, the Andes have caught the imagination of travelers, inspiring fear and wonder. The groundbreaking scientist Alexander von Humboldt claimed that everything here is grander and more majestic than in the Swiss Alps, the Pyrenees, the Carpathians, the Apennines, and all other mountains I have known. Rivaled in height only by the Himalayas and stretching more than 4,500 miles, the sheer immensity of the Andes is matched by its concentration...