A grotto (Italian grotta) is any type of natural or artificial cave that is associate 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.