The Grotta Gigante is the Carso’s main tourist attraction, and with good reason: it’s the world’s largest accessible cave, and the second-largest natural chamber anywhere in the world. As it’s 98m high by 76m wide, the dome of St Peter’s would fit comfortably inside. It’s a steady 11°C inside, so bring warm clothes.

The cave is impressive in scale and, like most of the caves in the Carso, was created by the erosive action of a river, in this case the Timavo, which sank deeper and deeper underground before changing course (the cave is now dry). The fantastically shaped stalactites and stalagmites were formed by deposits of calcium carbonate and colourful metal oxides. Much more recently, ferns and moss have started to grow in what was previously a lifeless environment, thanks to photosynthesis triggered by electric lighting. The two long “pillars” in the centre of the cave are in fact wires sheathed in plastic. At the bottom end two super-accurate pendulums are suspended, used to measure seismic shifts in isolation from surface noise and air currents.

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