Just south west of the island of Vis you will find the tiny island of Bisevo. This little island is special as it is home to the blue grotto. The blue grotto is a natural phenomenon as between 11 am and noon on a sunny clear day, rays of light passing through an underwater opening illuminate the cave with a luminous blue light, and objects beneath the surface shimmer in pink and silver.
It is a highlight of most crews' weeks, and we can't recommend visiting it enough. As it is so popular it can get very busy with yachts anchoring off, before heading past the cave 'gatekeeper' in their dinghy.
If you don't really fancy jostling with all the other contenders in the early morning, then there are trips from Vis which include a water taxi to Bisevo before heading in on an official 'Blue grotto' boat. We have often found this is the best experience.