In archaeology, rock art is human-made markings placed on natural surfaces, typically vertical stone surfaces. A high proportion of surviving historic and prehistoric rock art is found in caves or partly enclosed rock shelters; this type also may be called cave art or parietal art. A global phenomenon, rock art is found in many culturally diverse regions of the world. It has been produced in many contexts throughout human history. In terms of technique, the four main groups are: cave paintings, petroglyphs, which are carved or scratched into the rock surface, sculpted rock reliefs, and geoglyphs, which are formed on the ground. The oldest known rock art dates from the Upper Palaeolithic period, having been found in Europe, Australia, Asia, and Africa. Anthropologists studying these artworks believe that they likely had magico-religious significance. The archaeological sub-discipline of rock art studies first developed in the late-19th century among Francophone scholars studying the rock art of the Upper Palaeolithic found in the cave systems of parts of Western Europe. Rock art continues to be of importance to indigenous peoples in various parts of the world, who view them as both sacred items and significant components of their cultural heritage. Such archaeological sites may become significant sources of cultural tourism and have been used in popular culture for their aesthetic qualities. Etymology The term rock art appears in the published literature as early as the 1940s. It has also been described as "rock carvings", "rock drawings", "rock engravings", "rock inscriptions", "rock paintings", "rock pictures", "rock records", and "rock sculptures." Background Parietal art is a term for art in caves; this definition usually extended to art in rock shelters under cliff overhangs. Popularly, it is called "cave art", and is a subset of the wider term, rock art. It is mostly on rock walls, but may be on ceilings and floors. A wide variety of techniques have been used in its creation. The term usually is applied only to prehistoric art, but it may be used for art of any date. Sheltered parietal art has had a far better chance of surviving for very long periods, and what now survives may represent only a very small proportion of what was created. Both parietal and cave art refer to cave paintings, drawings, etchings, carvings, and pecked artwork on the interior of caves and rock shelters. Generally, these either are engraved (essentially meaning scratched) or painted,