Check this, I have included two of them!
In archaeology, rock art is human-made markings placed on natural stone; it is largely synonymous with 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, although the majority of rock art that has been ethnographically recorded has been produced as a part of ritual. Such artworks are often divided into three forms: petroglyphs, which are carved into the rock surface, pictographs, which are painted onto the surface, and earth figures, formed on the ground. The oldest known rock art dates from the Upper Palaeolithic period, having been found in Europe, Australia, Asia and Africa. Archaeologists studying these artworks believe that they likely had magico-religious significance.
There are three basic types of relief sculpture: (1) low relief where the sculpture projects only slightly from the background surface; (2) high relief where the sculpture projects at least half or more of its natural circumference from the background, and may in parts be wholly disengaged from the ground, thus approximating sculpture in the round. [Sculptors may also employ middle-relief , a style which falls roughly between the high and low forms]; (3) sunken relief, where the carving is sunk below the level of the surrounding surface and is contained within a sharpely incised contour line that frames it with a powerful line of shadow. The surrounding surface remains untouched, with no projections. Sunken relief carving is found almost exclusively in ancient Egyptian art, although it has also been used in some beautiful small-scale ivory reliefs from India.
I believe the answer is "Acoustics"
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