Sea cave famous for the spectacular blue reflections produced by sunlight refracting on the water inside, a phenomenon similar to that of better-known caves such as the one on Capri, though on a smaller scale. Reachable only by sea, it lies on the western side of the Circeo promontory, on a stretch of coast accessible mainly by small boat or canoe.
The evocative Blue Grotto owes its name to the wonderful iridescence the sea produces on its walls, in tones ranging from emerald green to deep blue with patches of violet. This variety of colours is due to the remains of plant life growing on the rock and to the reflections of the seawater, which is very clear here and gives an excellent view of the sandy bottom. The cave is not very large compared with the other caves of the promontory: it is about 30 metres long and 10 wide. Its height is rather low, and it can be entered only in calm seas and at low tide, by small boat or swimming. Inside, scholars have identified a series of straight horizontal tunnels, signs of mechanical erosion by material carried by the pressure of water masses: probably the final stretches of ancient watercourses. Near the cave there are also some springs, where rainwater absorbed by the rock comes back to the surface.
