The Blue Cave sits in Balun cove on Biševo, a six square kilometre island with about twenty permanent residents, five nautical miles south-west of Komiža. The chamber is roughly 24 metres long, 10 to 12 metres wide and up to 15 metres high, with water up to 16 metres deep beneath the boat. The entrance you duck through is about 1.5 metres high and 2.5 metres wide.
None of the blue comes from a lamp. Daylight enters through a submerged opening on the south side, refracts through the water and bounces off the pale limestone floor, so the chamber glows from below and anything under the surface turns silver. It is strongest on a calm, sunny morning, roughly between 11:00 and 13:00.
Until 1884 the only way in was to dive. That year an entrance big enough for a small boat was blasted in the rock, after the Austrian painter and naturalist Baron Eugen von Ransonnet documented the cave and argued for opening it up.
No private boat goes inside, ours included. Everyone transfers at Mezuporat cove onto the licensed cave boats and waits for a numbered ticket. The official limit is fifteen minutes inside; in practice you get five to ten. Swimming inside is not allowed.
WhereBalun cove, Biševo island
Chamberabout 24 m long, up to 15 m high
Entranceabout 1.5 m high, 2.5 m wide
Best lightroughly 11:00 to 13:00, calm and sunny
Time inside5 to 15 minutes, numbered ticket
Swimmingnot permitted inside the Blue Cave