The region of Friuli Venezia Giulia encloses in its underground a spectacular and mysterious world which winds its way through dark caverns and such extended cavities that are so big that they could hold the Basilica of Saint Peter’s: this is the case of the Giant Cave, which spreads out in the Carsic underground just behind Trieste. It is well-known as the largest cave in the world which may be explored (it is in actual fact 107 metres high and 280 metres wide), it is embellished by spectacular stalactites and stalagmites, some of which look just like palm trees and reach a height of even 12 metres.
The Villanova Caves are also unique. They are situated in the Alta Val Torre, and are made up of a network of cavities of which the most striking one is the Grotta Nuova – New Cave (in the Town District of Lusevera), as it is the largest contact cave between two rocky banks which may be visited in Europe. Its galleries, ploughed through by an underground torrent, wind their way over a surface of 7 kilometres, of which a part is illuminated and may be comfortably and easily visited. Together with a guide and special equipment supplied on site, visitors may dare to push themselves into the adventure of exploring the innermost part of the caves, on a trip into the heart of the earth which takes about four hours.
The Grotta Verde in Pradis (Town District of Clauzetto) is a national temple for speleologists. It is called the Green Cave because of the enchanting emerald green reflections on its walls: it is a jewel of the Pordenone mountains, and also a place of religious cult dedicated to the Madonna, represented by a bronze statue in whose presence the Holy Mass at Christmas is celebrated. 
An ancient place dedicated to the religious cult is also the Cave of San Giovanni d'Antro, in the valleys of the Natisone river. You enter the cave by climbing down a steep stairway immersed in the green surroundings and inside an imposing wooden altarpiece dating back to the seventeenth century may be admired..