Blue Grotto

The Blue Grotto is a sea cave on the island of Biševo in Croatia, whose white limestone walls shimmer in glowing blue. The interior extends 24 meters deep and 36 meters long, while a submerged opening filters sunlight through the water.

Austrian Baron Eugen von Ransonnet visited the cave in 1884 and published reports that drew European travelers to the natural phenomenon. In the following decades, a small entrance was carved into the rock to ease access for visitors.

Locals call the grotto Modra špilja and describe the turquoise glow with words that have evolved in the island dialect over decades. Fishermen often visit the cave early in the morning when the sea is calm and the sunlight creates the first reflections on the limestone walls.

Small boats depart from Komiža or Vis to reach the site, carrying only a few passengers at a time due to the low and narrow entrance. The light effect works best between 11 AM and noon, when the sun is high and shines through the underwater opening.

Swimmers and objects in the water transform into silvery shapes against the blue illuminated background through the filtered sunlight. The optical illusion happens because light passes through water twice before reaching the surface.