The Melissani Cave, near the town of Sami on Kefalonia's east coast, is a partly submerged lake inside a collapsed cave chamber. The visit takes place in a rowing boat, with a guide who steers you through the illuminated sections of the cave and under the open sky where the roof has fallen in.
The effect of the light on the water — particularly around noon in summer when the sun falls directly into the opening — is extraordinary: the lake turns a vivid, almost electric blue. The cave is divided into two chambers; one is entirely underground and more dimly lit, the other open to the sky. The whole visit takes about 15-20 minutes.
Buy tickets at the entrance; queues in high season can be long in mid-morning. Arrive early or after 3pm to avoid the worst of them.