The Cañón del Río Lobos Natural Park Nature · Ucero, Tierras del Burgo, Soria The Cañón del Río Lobos Natural Park, declared by decree 115/1985 of the Junta of Castile and León, protects the gorge that the Lobos river has carved into the Upper Cretaceous limestone, on the border between the provinces of Soria and Burgos, among the municipalities of Ucero and Hontoria del Pinar, among others. It covers 10,202 hectares of karst terrain with abundant caves, sinkholes and swallow holes, formed both by the wearing action of the river itself and by the dissolution of the rock. Its vertical walls, caves and meanders make it one of the most visited river canyons in Castile and León. Photo: Diego Delso (CC BY-SA 4.0) What to see and do in Ucero