Grotta di Su Marmuri

High above Ulassai, the mammoth Grotta di Su Marmuri is a 35m-high cave complex. Visits are by guided tour only (minimum of four people), which take you on a one-hour, 1km walk through an underground wonderland festooned with stalactites and stalagmites – some like humongous drip candles, others as delicate as coral. Whatever the temperature outside, it is always chilly down here, so be sure to bring some extra layers.