The Blue Cave on the neighbouring island of Biševo is the most famous thing about Vis and one of the most-photographed natural features on the Croatian coast. A sea cave with a small surface opening and a much larger submerged side entrance, it lights up in the late morning when angled sunlight bounces off the white limestone floor and turns the water a near-radioactive blue.
Visits run from Komiža harbour (small private boats, around 30 minutes each way), from Vis town as part of a day trip taking in multiple coves, or from Split and Hvar on bigger tours. The boats transfer you to small rowed dinghies at the cave entrance — the surface opening is too low for anything bigger to clear. Allow most of the day if you want to combine with stops at Stiniva and the green cave further south.