The center of everything is now the middle of nowhere: For 400 years through the mid-13th century, Chaco Canyon was the trade and political nexus of a vast Puebloan culture. These days the only way into Chaco is along rough tracks that are drivable, if slowly, under dry conditions. It’s tempting to go straight to the D-shaped Pueblo Bonito, which once housed 600 rooms. Instead hike up the 5.5-mile Pueblo Alto Loop for an overview of the great houses, their intricate masonry walls standing four stories tall. Chaco may be my favorite site, but the four that follow are remarkable, too.