Paul Salopek continues his walk through Shaanxi, China. First, Salopek and his walking partners splashed into Beijing in wet weather.

Then, from Beijing, he worked his way to Taiyuan, a stone-age site that is important because archeologists discovered the 40,000-year-old remains of an early human there.

In the same area, many rural villagers used to live in underground villages. Yet in recent years the vast majority have chosen to move above ground, and increasingly they've also moved on to the cities to find work.

"The grizzled inhabitants of the Yellow Earth Plateau have added an intermediary step in that mass evacuation: migrating upward first, then out. Liu and I hike past hundreds of abandoned cave houses. Some are padlocked. Others gape open to the bitter winds," Salopek wrote about the abandoned villages in a recent update on his social media.