The Blue Caves at Cape Skinari were formed over two million years by wave erosion carving into soft Cretaceous limestone, leaving a chain of collapsed sea arches and submerged chambers along Zakynthos's northern coast. The largest vault reaches twelve metres in height. Sunlight refracts through the shallow water and reflects off the pale seabed, saturating the caves in a blue so vivid that nineteenth-century sailors believed the cliffs themselves glowed. Geologists classify the formation as a littoral karst system. The rock's high calcium carbonate content interacts with seawater to create the prismatic effect visible only between late morning and early afternoon, when solar angle permits direct light penetration. Outside that window, the caves appear grey-green. The phenomenon draws over 200,000 visitors annually, most arriving by speedboat or traditional kaiki from Cape Skinari's small harbour. Commercial zakynthos boat tour blue caves operations began in the 1980s after asphalt reached the cape, replacing the donkey track that previously connected the peninsula to inland villages. The cave system extends along a 1.5-kilometre coastline. Three chambers permit full boat entry; four others require swimming access. Water temperature averages 24°C in summer, dropping to 16°C by November. Visibility exceeds fifteen metres on calm days, revealing rock formations and occasional loggerhead sea turtles that nest on nearby beaches. Tour operators anchor outside the largest cave to allow passengers to swim beneath the arches, a practice regulated since 2019 to prevent anchor damage to the seabed. The site remains free to access, though zakynthos boat tour blue caves tickets cover vessel transport, typically ranging from group cruises to private speedboat charters. Morning departures offer the calmest sea conditions and optimal light, critical factors given the caves' exposure to prevailing northwest winds.