Grotta dello Smeraldo, Sea cave in Conca dei Marini, Italy.

The Grotta dello Smeraldo is a partly sea-flooded karst cave in Conca dei Marini, on the Amalfi Coast. Its water surface measures about 45 by 32 metres, and the roof rises some 24 metres above the water. Stalactites and stalagmites, in places fused into columns over ten metres tall, stand partly submerged; the only opening to the sea lies below the waterline.

The cave formed when tectonic uplift fractured the cliffs; it later sank below sea level through bradyseism, flooding its stalagmites. With no opening above the waterline, it stayed unknown until the fisherman Luigi Buonocore found it in 1932. In 1956 a ceramic nativity scene was placed on its seabed, and divers still lay flowers there at Christmas.

The name comes from the emerald tint sunlight gives the water after passing through the submerged opening. Since 1956 a presepe of ceramic figurines has been laid on the seabed; each Christmas divers place bunches of flowers at the feet of the Baby Jesus. The grotto also served as a set for the TV series Capri.

Access to the cave is gained either by elevator descending from the coastal road or by boat tours departing from Amalfi. The best time to visit is during sunny hours when light penetrates most effectively into the interior.

The water inside glows with an intense green tone, created by sunlight entering through an underwater fissure in the rock. This optical effect shifts throughout the day as the sun's angle changes, making each visit visually different.