The underground karst (limestone) caves, formed by dripping surface water, were not properly explored until 1938, when they became the most celebrated in Italy. Tunnels, passageways, crystalline columns, stalactites, stalagmites, natural bridges and echoing vaults go on for 2 km, glowing and sparkling in artificial light and bursts of sunlight from apertures high above where the roof has fallen in. Coloured rocks evoke a response from the most blasé cave-explorer. Open daily, all year round. The full tour takes about three hours.