In many languages, the word grotto is used for all kinds of caverns. In French and Italian it is used for natural caves. In the Italian part of Switzerland it is used for wine bars. In English the term cave means a natural cavern, while a grotto is an artificial cavern. However, there are old names and old literature which do not use the term in this strict manner. A common use of the term was to describe a rather small cavern with abundant ornamentation, both natural or artificial. The last part is actually where it is still used today.
The grottoes, we talk of on this page, are a special sort of artificial cavern, typically built during the 18th and 19th century. They are similar to the Britsh shell houses, both are artificial caverns, or better buildings which resemble ruined castles, which generate an underground atmosphere inside, are decorated with shells, rocks, and minerals. There are even grottoes with fountains inside.
Another special form of grottoes is the Lourdes-grotto. The original Lourdes grotto is a small natural cave with a spring in the village Lourdes, where a young woman had visions of the Virgin Mary. It is a catholic pilgrimage site and decorated like a church. Lourdes grottoes are more or less replicas of the original grotto, in general completely artificial, Most of those grottoes are similar to altars or shrines. They are generally not listed on this website.