As a teenager, Kate Harris wanted to be an explorer, equal parts swashbuckler and metaphysician. With the Earth already mapped by the likes of Magellan and Marco Polo, she set her sights on the stars: she would become a scientist and go to Mars. But in between studying at Oxford and MIT, she set off by bicycle with a childhood friend down the fabled Silk Road, where, pedaling mile upon mile through some of the remotest places, she realized that exploration—in any day or age—is the refusal to liv...