There are five pyramids. They are named after the Pharaohs who built them: the Pyramid of Chephren, the Pyramid of Mycerinus, the Pyramid of Menkaura, the Pyramid of Kha and the Pyramid of Narmer. The first four were built between 2613 and 2504 B.C. and are made of limestone. The last one was built later on; it's made of granite and dates back to 2494 B.C. All together, these six pyramids are among the earliest monumental structures in Egypt. The design of all six is similar, with a square base followed by a steep-sided pyramid with flat top and straight sides. There are no decorations except for the carving on some of the stones on the pyramids at Saqqara. Modern researchers believe that all the pyramids were built by the same team of architects over a period of about 20 years, but this cannot be proven yet because none of the original documents have been found. Some historians think that some of the pyramids may actually be tombs belonging to high-ranking officials while others are probably just large stone markers showing that someone important lived here at one time.