Bisevo Blue Cave

Of all the sea caves on the Croatian coast, one has become a fixture on almost every Dalmatian itinerary. The Blue Cave on the small island of Biševo, off the western end of Vis, does something the others cannot: for a couple of hours in the middle of a calm, sunny day the whole chamber turns an improbable electric blue, lit entirely from below.

How the blue light works

The Blue Cave (Modra Špilja) on Biševo Island is one of the most-visited natural sights in this part of the Adriatic, and for good reason — between roughly 11am and 1pm on a calm, sunny day, sunlight passing through an underwater opening bounces off the white limestone floor and fills the cave with a bright blue glow, while anything submerged takes on a silvery sheen.

The mechanism is straightforward. A natural opening on the southern side of the cave sits below the waterline; sunlight refracts through it, reflects off the pale limestone bottom and rises back into the chamber having shed its red wavelengths on the way, which is why the light reads blue rather than white. The chamber itself is modest, put at around 24 metres long and up to 15 metres high, with ten to twelve metres of water beneath the boat. It was cut by the sea itself, wave action steadily dissolving the limestone that makes up the whole of Biševo.

A cave that had to be broken into

The cave was originally accessible only by diving through its submerged entrance; a small artificial opening was cut in 1884, which is what lets boats — small rowboats or dinghies, since the entrance is too narrow and low for anything bigger — get inside today.

The push for that opening came from Baron Eugen von Ransonnet, an Austrian painter, diplomat and pioneer of underwater illustration, who described and painted the cave and argued that it deserved to be seen. The entrance made on his recommendation measures only about 1.5 metres high and 2.5 metres wide, which is why the transfer boats are so small and why everyone aboard is asked to duck. Before that, seeing the interior meant free-diving through the submerged gap, and the place was known mainly to Komiža fishermen.

Biševo itself

The island the cave belongs to is tiny: about 5.8 square kilometres, rising to 239 metres at Stražbenica and lying roughly five kilometres southwest of Vis. Its permanent population was recorded as 15 at the 2011 census, though the figure swells through the summer. A Benedictine monastery was founded here in 1050 by Ivan Grlić of Split; the church of St Sylvester survives beside its ruins, and the community was eventually abandoned after repeated pirate raids. Vines and fishing kept the island going for centuries afterwards, the school opened in 1947 and closed in 1961, and mains electricity only arrived in the 1970s.

Biševo has more than one cave. The Monk Seal Cave on its southern side runs about 160 metres into the rock, making it the longest sea cave in the Adriatic, and takes its name from the Mediterranean monk seals that once sheltered there; it has been protected as a geomorphological monument of nature since 1967. Salbunara, a rare stretch of genuine sand, is the best swimming bay, and the little harbour at Mezuporat is where cave tickets are sold.

Protection, tickets and the rules

Since 17 April 2019 the whole Vis Archipelago — Vis, Biševo, Sveti Andrija, Brusnik, Jabuka and Palagruža — has carried UNESCO Global Geopark status, the second such designation in Croatia. Entry to the cave is ticketed, the fee covering the short transfer aboard the official small boats rather than the passage out from Komiža, and prices differ between peak and shoulder season. Time inside is deliberately brief, a matter of minutes per group, to keep the queue moving and limit engine fumes and wash in the chamber.

The cave is only open May through October, weather and sea conditions permitting, with visits generally running from around 9am to 5pm. Excursion boats leave Komiža harbour early in the morning to reach Biševo in time for the best light, and swimming inside the cave itself isn't allowed, both to protect the site and to keep the narrow entrance clear for other boats.

Working it into a cruise

Komiža lies about 4.5 nautical miles from Biševo, which is why the town is the natural jumping-off point; from the mainland, the ferry from Split to Vis takes roughly two hours twenty minutes, or about ninety minutes by catamaran. The largest variable is the sea. A southerly swell closes the cave outright and even moderate chop makes the entrance unworkable, so it pays to keep the day flexible rather than pin a whole itinerary on it.

Most people see the cave as part of a wider stop around Biševo rather than a destination on its own — it's one of the anchorages you'll visit on a private gulet cruise in Croatia, usually combined with time in the surrounding bays.