The Carso's big-ticket attraction is this enormous underground cavern near the town of Opicina. At 120m high, 280m long and 65m wide, it's one of the largest caves in the world, filled with stalactites and stalagmites. Knowledgeable guides point out the cave's features, which include two huge pendulums that monitor the movement of the earth's crust. Upstairs there's a small Speleological Museum, which includes fossils and objects found in the cave, including the skeleton of a now-extinct karst bear.
To reach the cave from Trieste take bus 42, or tram 2 and bus 42 in the other direction.