A Grotto (Italian grotta) is any type of natural or artificial cave that is associated with modern, historic or prehistoric use by humans. When it is not an artificial garden feature, a grotto is often a small cave near water and often flooded or liable to flood at high tide. The picturesque Grotta Azzura at Capri and the grotto of the villa of Tiberius in the Bay of Naples are outstanding natural seashore grottoes. Whether in tidal water or high up in hills, they are very often in limestone geology where the acidity dissolved in percolating water has dissolved the carbonates of the rock matrix as it has passed through what were originally small fissures. See karst topography, cavern.

At the great Roman sanctuary of Praeneste south of Rome, the oldest portion of the primitive sanctuary was situated on the next-to-lowest terrace, in a grotto in the natural rock where there was a spring that developed into a well. Such a sacred spring had its native nymph, who might be honored in a grotto-like nymphaeum, where the watery element was never far to seek.

Tiberius filled his grotto with sculptures to recreate a mythological setting, perhaps Polyphemus' cave in the Odyssey. The numinous quality of the grotto is still more ancient, of course: in a grotto near Knossos in Crete, Eileithyia had been venerated even before Minoan palace-building, and farther back in time the immanence of the divine in a grotto is an aspect of the sacred caves of Lascaux.

The word comes from Italian grotta, Vulgar Latin grupta, Latin crypta, (a crypt). It is related by a historical accident to the word grotesque: in the late 15th century, Romans unearthed by accident Nero's Domus Aurea on the Palatine Hill, a series of rooms underground (as they had become over time), that were decorated in designs of garlands and animals. The Romans found them thought them very strange, a sentiment enhanced by their 'underworld' source. Because of the situation in which they were discovered, this form of decoration was given the name grotesque. http://en.wikipedia.org/wiki/Grotesque