Grotte di Ulldecona

Ulldecona, a municipality in the Montsià, has 27 catalogued cavities: 18 caves and 9 pits. Together they total a length of 761 m and reach a maximum depth of 58 m. All the cavities develop in limestone.

Among the cavities most notable for their depth are the Avenc de l'Arion, about 58 m, explored in 1934 and relocated in 2015 by the EC Tortosa; the Avenc dels Castellets, about 52 m, explored since the 1960s and surveyed to about 40 m between 1980-83; and the Avenc de les Senioles, 21 m, with a single room of notable dimensions and an important colony of chiropterans. As for length, the Avenc de les Canals and the Avenc dels Castellets each reach 80 m, followed by the Avenc de l'Arion with 70 m and the Cova Bonica with 65 m.

Other cavities of interest are the Cova de Santa Magdalena, 56 m long and of archaeological interest, with an engraving of an anthropomorphic halteriform, and the Avenc de Julio de la Sista, 17 m deep, with a narrow entrance, a 10 m shaft and a room with formations, visited in December 2018.