The Grotta del Bue Marino is the most celebrated of the sea caves on the western side of San Domino, accessible only by boat. The cave entrance is large enough for a small vessel to enter, and inside the light plays through the water in extraordinary ways — blues and greens shifting with the movement of the sea, the cave walls appearing to glow. Boat trips that circle the island will pass or enter several caves; this is the largest and most visited.
The best time to visit the cave interiors is mid-morning when the sun angle is right for maximum light penetration. Tour boats depart from the San Domino dock throughout the summer season; you can also rent kayaks and paddle to the caves at your own pace, which gives you more time and more freedom to stop where you want. The coastline of San Domino is riddled with smaller inlets and coves worth exploring.