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 following way: 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, slender architectural framework, foliations, and animals. The Romans who 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 grottesche or grotesque.
A grotto (Italian grotta and French "grotte") 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 Azzurra 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.
In modern times, many people purchase artificial grottos for ornamental and devotional purposes when it comes to placing statues of saints, particularly the Blessed Virgin, in outdoor gardens. The well-known apparitions of Our Lady of Lourdes to Bernadette Soubirous took place in a grotto, which is visited by many Catholics. Numerous garden shrines are modeled after these apparitions, and can commonly be found displayed in gardens and Churches, among other places (see Lourdes grotto).
Mantic springs that issued from grottoes were a feature of Apollo's oracles at Delphi, Corinth, and Clarus. The new-built Hellenistic city of Rhodes was provided with rock-cut artificial grottos with "naturalistic" features. 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.