Ruta 68 between Santiago and Valparaíso is one of Chile’s busiest highways, but every December 8 motor vehicle traffic miraculously vanishes as up to half a million faithful congregate at the Santuario de la Inmaculada Concepción, one of official Catholicism’s holiest Chilean shrines.

After the 1906 Valparaíso earthquake destroyed the original chapel, it took more than 30 years to build the present church, dating from 1940. The 4,000-square-meter amphitheater can accommodate nearly 3,000 worshippers.

To reach the shrine, 26 kilometers southeast of Valparaíso and 84 kilometers northwest of Santiago, pedestrian pilgrims literally take over the highway. Many start days earlier from Santiago and even farther away, sleeping outdoors en route; some even crawl the final five or six kilometers.

On arrival, they attend open-air masses, which take place from 6 p.m. the afternoon of the December 7 until 8 p.m. the evening of the 8th; the festivities close with a procession led by the bishop of Valparaíso. In the interim, authorities divert Valparaíso-and Viña-bound traffic north through Ruta 5 and Ruta 60 to Quillota.

Throughout the event, hawkers sell prodigious quantities of food and souvenirs—though there are almost-permanent eateries and the sanctuary has its own souvenir shop—and pilgrims deposit up to 20 tons of trash. At other times, they leave small monetary offerings in hopes of avoiding traffic accidents or perhaps speeding tickets, as Carabineros patrol this stretch of highway zealously.