Grotto , (It. grotto), a natural cavern, or an artificial excavation in the earth. Among the most famous caverns particularly designated by this name is the Kasegrotte at Bertrich, Rhenish Prussia, so called from the columnar piles of blocks of basalt shaped like cheeses, in the midst of which the cave extends. Still more celebrated is the Grotta del Cane, near Pozzu-oli, Naples, referred to by Pliny as one of the class of excavations known as "Charon's ditches." it would seem from his reference that in his time the mephitic gas for which it is still remarkable was exhaled in quantity sufficient to prove fatal to human life. At the present time this forms but a shallow stratum upon the floor, in which a candle is extinguished and dogs are stifled by way of experiment. The custom of exhibiting the effect of the carbonic acid gas upon dogs has given the distinctive name to the grotto. The excavation is described as extending about 10 ft. into the base of a hill, with a width of 4 and a height of 9 ft. Prof. Silliman, in his "Notes on Europe," speaks of it as a little hole dug artificially into the foot of a hill facing Lake Agnano. The aperture is closed by a door, and the space within is barely sufficient for one person to stand erect.
Into this narrow cell a dog is dragged, and placed in a depression of the floor, where he is soon narcotized by the carbonic acid. The earth is warm to the hand, and the volume of gas given out is very constant. Among other celebrated grottoes is that of the island of Antiparos. (See Axtiparos)