Italy Guide
Friuli-Venezia Giulia
Grotta Gigante
One of the Carso's highlights, the Grotta Gigante Opening time: guided visits daily: April– Sept every 30min 10am–6pm; March & Oct hourly 10am–4pm; Nov– Feb hourly 10am– noon & 2–4pmPrice: €8.50 
www.grottagigante.it is the world's largest accessible cave, and the second-largest natural chamber anywhere in the world (the largest is in Malaysia). As it's 107m deep by 208m broad, the dome of St Peter's would fit comfortably inside. It's a steady 11°C inside, so come prepared.
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 stalactites and stalagmites grew later, 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.