Petroglyphs are rock carvings made by removing part of a rock surface by incising, picking, carving, or abrading, as a form of rock art. The word "petroglyph" comes from the Greek prefix petro-, meaning "stone", and γλύφω glýphō, meaning "carve". Petroglyphs are found worldwide and are often associated with prehistoric peoples. They are estimated to be 20,000 years old and are classified as protected monuments. Petroglyphs are often created by pecking directly on the rock surface using a stone chisel and a hammerstone. When the desert varnish or patina on the surface of the rock is chipped off, the lighter rock underneath is exposed, creating the petroglyph. Petroglyphs are different from pictographs, which are rock paintings. Some of the oldest petroglyphs are the Bhimbetka Petroglyphs in India, which were created between 290,000 and 700,000 BCE. The purpose of petroglyphs is not always clear, but many images are thought to represent a type of symbolic or ritualistic language or have deep cultural and religious significance for the societies that created them.