A grotto (Italian grotta and French grotte) is a natural or artificial cave used by humans in modern times, historically or prehistorically. Sometimes used as an artificial garden feature, a naturally occurring grotto is often a small cave near water and often flooded or liable to flood at high tide. The Grotta Azzurra at Capri and the grotto at the villa of Tiberius in the Bay of Naples are popular natural seashore grottos.

Whether in tidal water or high up in hills, grottos are often made up of limestone geology where the acidity of standing water has dissolved the rock matrix' carbonates as it passes through what were originally small fissures. See karst topography, cavern.

Today, artificial grottos are purchased for ornamental and devotional purposes. They are often used as places to place statues of saints, particularly the Virgin Mary, in outdoor gardens. The 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).

Springs that were fed by grottoes were a feature of Apollo's oracles at Delphi, Corinth, and Clarus.^[1] The Hellenistic city of Rhodes was designed with rock-cut artificial grottos made to look natural.^[2] At the great Roman sanctuary of Praeneste south of Rome, the oldest portion of the primitive sanctuary was situated on the second lowest terrace, in a grotto in the natural rock where a spring developed into a well. This sacred spring had a native nymph, who was honored in a grotto-like watery nymphaeum.^[3]

Tiberius filled his grotto with sculptures to create a sense of mythology, perhaps channeling 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).^[4] 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 that had sunk underground over time that were decorated in designs of garlands, slender architectural framework, foliations, and animals. The Romans who discovered them found it very strange, a sentiment enhanced by the fact that they were uncovered from an 'underworld' source. This strange situation led the Romans to give ti the name "grottesche" or grotesque.

The popularity of artificial grottoes introduced Mannerist style to Italian and French gardens of the mid 16th century. Two famous grottoes in the Boboli Gardens of Palazzo Pitti were begun by Vasari and completed by Ammanati and Buontalenti between 1583 and 1593. One of these grottoes originally housed the Prisoners of Michelangelo. Even earlier than the Boboli grotto, a garden was laid out by Niccolò Tribolo (died 1550) at the Medici Villa Castello, near Florence. At Pratolino, in spite of the dryness of the site, there was a Grotto of Cupid (surviving), with water tricks for the unsuspecting visitor.^[5] The Fonte di Fata Morgana ('Fata Morgana's Spring') at Grassina, not far from Florence, is a small garden building, built 1573–4 as a garden feature in the extensive grounds of the Villa "Riposo" (rest) of Bernardo Vecchietti. It is decorated with sculptures in the Giambolognan manner.

The outside of garden grottoes are often designed to look like an enormous rock, a rustic porch or a rocky overhang. Inside, they're decorated as a temple or with fountains stalactites and imitation gems and shells (sometimes made in ceramic); herms and mermaids, mythological subjects suited to the space: naiads, or river gods whose urns spilled water into pools. Damp grottoes were cool places to retreat from the Italian sun, but they also became fashionable in the cool drizzle of the Île-de-France. In Kuskovo at the Sheremetev estate there is a Summer Grotto, built in 1775.

Grottoes could also serve as baths, for example at the Palazzo del Tè,in the 'Casino della Grotta', a small suite of intimate rooms laid out around a grotto and loggetta (covered balcony), courtiers once bathed in the small cascade that splashed over the pebbles and shells encrusted in the floor and walls.

Grottoes have served as chapels, or at Villa Farnese at Caprarola, a little theater designed in the grotto manner. They were often combined with cascading fountains in Renaissance gardens.

The grotto designed by Bernard Palissy for Catherine de' Medici's château in Paris, the Tuileries, was renowned. In the gardens designed by André Le Nôtre for Versailles there are also grottos. In England, an early garden grotto was built at Wilton House in the 1630s, probably by Isaac de Caus.

Grottoes were suitable for less formal gardening too. Pope's Grotto, created by Alexander Pope, is almost all that survives of one of the very first landscape gardens in England, at Twickenham.^[6] There are grottoes in the famous landscape gardens of Painshill Park,^[7] Stowe, Clandon Park and Stourhead.^[8] Scott's Grotto is a series of interconnected chambers, extending 67 ft into the chalk hillside on the outskirts of Ware, Hertfordshire. Built during the late 18th century, the chambers and tunnels are lined with shells, flints and pieces of coloured glass.^[9] The Romantic generation of tourists might not actually visit Fingal's Cave, located in the isolated Hebrides, but they have heard of it, perhaps through Felix Mendelssohn's "Hebrides Overture", better known as "Fingal's Cave," which was inspired by his visit. In the 19th century, when miniature Matterhorns and rock-gardens became fashionable, a was often found, such as at Ascott House. In Bavaria, Ludwig's Linderhof contains an abstraction of the grotto under Venusberg, which figured in Wagner's Tannhäuser.

Although grottoes have largely fallen from fashion since the British Picturesque movement, architects and artists occasionally try to redefine the grotto in contemporary design works. Such examples include Aranda/Lasch's Grotto Concept, (2005), Callum Moreton's Grotto pavilion (2010), ARM's post-modern Storey Hall (1995), and Frederick Kiesler's Grotto of Meditation for New Harmony (1964).^[10]

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  Grotto in an iceberg with the Terra Nova ship in the background (1911) during the 'British Antarctic Expedition' by Herbert G. Ponting
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  Grotto of the Buttes-Chaumont in Paris.
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  Grotto pavilion in Kuskovo, Moscow (1775).
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  Grotto entrance, Villa Torrigiani
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  Sculpture in a grotto setting, Villa Torrigiani, Lucca
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  Anapat Grotto, Lastiver, Armenia
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  Shell Grotto, Wales

• Jackson, Hazelle (2001). Shell Houses and Grottoes. England: Shire Books). Traces the development of the grotto in Italy during the Renaissance and its popularity in the UK from the eighteenth century to the present. Includes gazetteer of UK grottoes.
• Jones, B. (1953). Follies and Grottoes. London.{{cite book}}: CS1 maint: location missing publisher (link)
• Miller, Naomi (1982). Heavenly Caves: Reflections on the Garden Grotto. New York: Braziller. Traces the development of the grotto from Antiquity to modern times.

• Blue Grotto, former underground wine storage vaults in the anchorages under the Brooklyn Bridge, on the Manhattan side
• Grotto-heavens, Chinese religious usage associated with Daoist religion
• Shell grotto