The pilgrimage began with a discovery -- or a legend, depending on how strictly one reads the evidence. In the early 9th century, a hermit named Pelayo reported seeing mysterious lights over a field in Galicia. The local bishop, Theodemir, investigated and declared that the remains of the Apostle James the Great had been found. King Alfonso II of Asturias walked from Oviedo to venerate the relics, becoming the first recorded pilgrim and establishing what is now called the Camino Primitivo, the Primitive Way. A church was built over the tomb, then a cathedral, then a city: Santiago de Compostela, named for the "field of stars" where the bones were said to rest. By the 10th century, pilgrims were arriving from across Europe in numbers that transformed the economy and culture of northern Spain.

The most popular route is the Camino Frances, which begins where four feeder routes from France merge after crossing the Pyrenees at Roncesvalles and Somport Pass. From there, it runs 738 kilometers west through five autonomous communities -- Aragon, Navarre, La Rioja, Castile and Leon, and Galicia -- and 166 towns and villages. The UNESCO inscription lists nearly 2,000 associated structures along this route alone: Romanesque churches with carved tympanums, medieval hostels built to shelter pilgrims, stone bridges spanning rivers that have shifted course since the 12th century, and wayside crosses marking where the path turns. The French Way is flat enough -- mostly following the meseta across central Spain -- that it became the dominant route once Christian kingdoms pushed the Muslim frontier southward in the 11th century, making the interior safe for travelers.

The Northern Way, or Camino del Norte, runs more than 930 kilometers along the Bay of Biscay coast from the French border to Galicia, passing through Irun, San Sebastian, Bilbao, Santander, and Gijon. It is the oldest of the routes after the Primitive Way, used by early pilgrims who could not risk crossing Muslim-held territory inland. The terrain is dramatic but punishing: coastal cliffs, steep ascents, and Atlantic weather that can turn a sunny morning into a drenching afternoon. The Interior Way, also called the Basque Way, offered a compromise -- heading southwest from Irun through the Oria valley and over the San Adrian tunnel to connect with the French Way at Santo Domingo de la Calzada, covering under 200 kilometers. The shortest route, the Liebana, runs just 55 kilometers to the Santo Toribio de Liebana Monastery, itself a pilgrimage site.

What distinguishes the Camino from a long-distance hiking trail is the infrastructure of hospitality that has grown along it for a millennium. The albergues -- hostels offering basic accommodation to pilgrims -- descend directly from medieval pilgrim hospitals. Churches along the route were built with pilgrims in mind: large enough to shelter crowds, oriented to catch the dawn light through their eastern windows, decorated with carvings that told biblical stories to travelers who could not read. The routes shaped the architecture, and the architecture shaped the routes. Today, hundreds of thousands of people walk some portion of the Camino each year, motivated by faith, fitness, curiosity, or some combination. The Pilgrims' Office in Santiago de Compostela issues a certificate of accomplishment -- the Compostela -- to anyone who walks the last 100 kilometers or cycles the last 200.

Most UNESCO sites are preserved places, frozen in a particular historical moment. The Camino is different. It is still in active use, still evolving, still producing the encounters between strangers and landscape that have defined it since the 9th century. New albergues open. Old churches are restored. Waymarking improves. The yellow arrows painted on walls, trees, and fence posts -- the informal but universal trail blazes of the Camino -- are refreshed every few years by volunteers. The 2015 UNESCO expansion recognized this dynamism by adding the northern routes and 16 individual structures of particular cultural significance to the original French Way inscription. The routes are not artifacts. They are paths still being walked, still wearing grooves into the stone of mountain passes, still carrying people toward a cathedral whose bells have been ringing since before the Normans crossed the English Channel.