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 in the world. As it’s 98m high by 76m wide, the cave is large enough that 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.

Book through Rough Guides’ trusted travel partners

Italy features

The latest articles, galleries, quizzes and videos.

13 striking pictures of Venice Carnival

13 striking pictures of Venice Carnival

With the 2018 carnival season about to kick off, Rough Guides author and photographer, Kiki Deere, revisits one of Europe's most colourful festivals: Venice…

25 Jan 2018 • Kiki Deere insert_drive_file Article
20 picturesque Christmas destinations

20 picturesque Christmas destinations

Dreaming of a trip this Christmas? We're here to help. From glittering cities to a few snowy escapes, here are 20 classic destinations for a festive break. …

12 Dec 2017 • Rough Guides Editors camera_alt Gallery
Discovering Palermo: why you should give Sicily’s capital a chance

Discovering Palermo: why you should give Sicily’s capital a chance

A city of grit and forgotten grandeur, for too long Palermo has resigned itself to being a film set of crumbling palaces, sun-worn facades and pockmarked backst…

08 Dec 2017 • Olivia Rawes insert_drive_file Article
View more featureschevron_right